BEFORE WE BEGIN
Yes, even though I have already posted a video on this I have changed my mind.
None of this review is opinionated except for the specific parts which are designed for my feedback.
If you have hate comments about my review, just remember it's only my opinion and you can have different opinions about each of these perks. I am speaking as a veteran player and even I have not found the correct balance for the perks.
Read every part of each perk before you start asking questions. This review should answer every single question, except for extra cases on parameter boosts -- we can't test every single case, just the important ones.
If you have anything to contribute to this page, leave a comment.

UPDATES:
28/5/14 - C4 damage was nerfed from 35 to 19. The blast radius was also reduced.

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Definitions:

[Picture]
Name: An unofficial name to describe the generic appearance of the perk
Use: The effect of the perk on the player, according to statistical evidence
Price: The price of the perk as shown on the Armory
Refresher: The time taken for the perk to become reusable after it is used.
Damage [If applicable]: The damage inflicted by the perk whilst no other perks are in effect
Crouch Damage [If applicable]: The crouch damage inflicted by the perk whilst no other perks are in effect
Evidence: Statistical evidence proving the use
Pros: Advantages
Cons: Disadvantages
Useful game modes: A personalised list of game modes which I believe the perk is most suited to.
Comment: Opinionated statement about the uses and tactics of the perk
Usefulness: Numerical data summarising the Comment out of 10.

Distinguishing between usefulness and rating: I cant simply give the C4 a usefulness of 1 because I hate it. This would be discrediting its use on the battlefield entirely and would bring the entirely wrong meaning for anyone reading this review. I've given it a rating based on its relative usefulness to other perks. This is intended to be a totally unbiased guide along with my personal opinion.

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Name: Medical Kit
Use: Maximum recovery of 10 HP at 1HP/second in between periods where the player suffers damage. Lasts for 10 seconds.
Price: 100 Potatoes
Refresher: 20 seconds
Damage [If applicable]: NA
Crouch Damage [If applicable]: NA
Evidence: I can recover 10HP after being shot, take an additional 1 or zero damage afterwards, and recover another 10HP.
Pros: Can be used for unlimited recovery given that you are careful enough about taking hits.
Cons: Can only be used to heal a VIP if they have fallen below 40HP. Very slow recovery. Works on both teams.
Useful game modes: Defend the flag, Domination
Comment: This is a perk I feel is an excellent choice for battles with extended standoffs. It's also an excellent choice when you are able to stall for time.
Usefulness: 7/10



Name: "Blue Laser"
Use: Reduction of received damage by approximately 20% for 10 seconds.
Price: 100 Potatoes
Refresher: 20 seconds
Damage [If applicable]: NA
Crouch Damage [If applicable]: NA
Evidence: Crouch damage of a C4 reduced from 11 to 10. Base damage of C4 reduced from 19 to 16. M40 A5 damage reduced from 17 to 14.
Pros: Gives you a slight defensive edge over the enemy in approximately equal standoffs.
Cons: The slight defensive boost doesnt really help when the rate of received damage is high. Gives boosts to both teams. Cannot be stacked - 20% is the maximum parameter increase.
Useful game modes: Defend the flag, Domination, VIP
Comment: Unfortunately I dont see much point to carrying this perk around except as a supportive boost. The 20% reduction in damage is very little and not very worth it (especially if you're confined to a small circle to keep the boost!!).
Usefulness: 4/10



Name: "Skull on a stick"
Use: Increase in damage inflicted to the enemy by approximately 15% for 10 seconds.
Price: 100 Potatoes
Refresher: 20 seconds
Damage [If applicable]: NA
Crouch Damage [If applicable]: NA
Evidence: Crouch damage of C4 rose from 11 to 13. Base damage of C4 increased from 19 to 21. M40 A5 damage rose from 17 to 19 [and a half].
Pros: Gives you a slight offensive edge over the enemy in approximately equal standoffs. An extremely good compliment for C4.
Cons: The slight offensive boost doesnt really work well with rapid firing weapons such as a FAMAS. Works on both teams. Cannot be stacked - 15% is the maximum parameter increase.
Useful game modes: Attack the flag, Defend the flag, Domination, VIP
Comment: Just a little better than its cousin. Kinda useless now because C4 was nerfed. The only way I can see it being useful is for group battles, and for landmines if you expect someone to stand on one in the next 10 seconds -- since it's instant kill.
Usefulness: 5/10



Name: C4
Use: A time bomb that explodes on the player's button press after 2 seconds. Blast diameter is damaging for 19 squares. Same damage regardless of distance from the centre.
Price: 100 Potatoes
Refresher: 10 seconds
Damage [If applicable]: 19
Crouch Damage [If applicable]: 11
Evidence: I measured the blast radius by conjuring one square at a time from the exact centre of the explosion.
Pros: Devastates tightly packed groups. Good as a scare tactic. Extremely good complimented with the shield perk. Friendly to other teammates. Can be used to travel long distances extremely quickly.
Cons: Skill drain (Relying too much on C4 can diminish your skill with a gun!!) Tendency for people to abuse it. Damaging to yourself. Only one at a time.
Useful game modes: Attack the flag, Defend the flag, VIP
Comment: I used to have an intense hatred for these things. However, when faced with a C4 planted in your base, you should just adjust your formation, or put up a shield. Often people tend to guess and hope that the C4 scores a kill even from a distance. By far the most common placement of these things is in the base. Another useful placement is in the upper void making it impossible for enemy players to reach you.
UPDATE: C4 has been nerfed. It's now less annoying, but also less effective to use.
Usefulness: 7/10



Name: Land Mine
Use: A motion sensor bomb that explodes when an enemy player steps on it. Blast diameter is damaging for 17 squares. Same damage regardless of distance from the centre.
Price: 100 Potatoes
Refresher: 10 seconds.
Damage [If applicable]: 35
Crouch Damage [If applicable]: 21
Evidence: Same as C4. I also personally checked if stacking was possible and it isnt.
Pros: Extremely good for restricting movement and blocking off tunnels. Team friendly. Is now better than C4 damage wise.
Cons: Smaller blast radius than the C4. Damaging to everyone if the mine goes off near anyone. Only one at a time.
Useful game modes: Defend the flag, Domination, VIP
Comment: You can use a C4 to disguise a landmine as the C4's sprite has priority over the landmine. Also remember that you can change perks in the middle of a game - and 10 seconds refresher is pretty worth your time to lay down one extra set of defense. Extremely good for preventing "void walkers" without having to look there yourself. You should make use of both landmines and C4.
Usefulness: 8/10



Name: Protective Shield
Use: Inflicted and received damage is reduced by 10 times for 5 seconds.
Price: 100 Potatoes
Refresher: 30 seconds.
Damage [If applicable]: NA
Crouch Damage [If applicable]: NA
Evidence: Blowing myself up with a C4 did zero damage (because I inflicted and took damage, 100 times less damage). M40 A5 did 2 damage without crouching. A C4 did 4 damage to me.
Pros: Extremely good for tanking hits as the VIP or if you want to use your own body as a shield. Not only that, you can run up to enemies even if their bullets push you back if you push yourself forward with "conjure".
Cons: You cant really do any damage yourself. You have to wait for the refresher timer to hit 25 before you use a c4 otherwise it'll just do 4 damage to the enemy. Long refresher time.
Useful game modes: Attack the flag, Defend the flag, Domination, VIP
Comment: I find myself always equipping this perk. It's saved my life plenty of times. Even though 30 seconds seems like a long time you'll probably make a huge difference just using it once. Even though not being able to do damage with this perk seems bad, it's especially good for breaching through heavily guarded spots (such as a base or VIP frontline)
Usefulness: 9/10

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BEST PERK COMBOS
Shield+Medic - Good for tanking hits and healing all of your HP if you are low
Shield+C4 - Great for rushing into a tight field of enemies, planting a C4 and then detonating it when the shield timer hits 25
C4+Landmine? - Excellent for gaining instant kills on enemies (as the attack increase raises the damage to beyond 40)
Laser+Skull - Excellent for powering up your entire team (especially if bunched up). Not too good by yourself.

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CREDITS AND ACKNOWLEDGEMENTS
Credit to Arber for inspiring the foundations for this review.
Credit to Fortoresse, Arber or any other source for providing the images for this review.
Credit to the game developers for bringing us this game!
Credit to the Gunpoint tribe for helping me test some of the data.
Names are not official but rather to identify each individual perk by appearance or generic name.

LAST UPDATED 29/5/14

This is a good review, but I would like to make a correction. If you are crouching when you get hit by a C4, you take 21 damage, not 19.

19 is what you're left with if you were at Max HP.

Ok that explains the little contradiction with the laser bringing 19 down to 17 and 17 down to 14. Not only that, it seems to show that the laser gives a bigger defensive boost than does the skull with its attack increase. Thanks. Fixed.

Snipers can do 1 - 2 damage to a person with protective shield, but c4 does nothing e__e

At least snipers have a long delay between each shot, and it usually takes 3~4 shots to kill someone. So with the shield how many shots a sniper takes to kill you originally only does 4~8 damage, which is quite good.

Incorrect Jjaakkee. "A C4 did 4 damage to me."

I don't see enough Attack+ perks. It's actually a pretty good perk. If a team would work together and place perks in one general area during a crossfire they'd be pretty much unstoppable unless the other team also does it.

If a team were to place: MedKit, Attack+, and Defense+
They'd be pretty stronk.

All that with human shields sponging up all the damage and you really would be unstoppable. It's just really tought to set stuff like that up without having a dedicated team (and a team that doesn't specificly use c4) even though you'd only really need two people to get it started.

I updated the info in the review to reflect the changes to C4. Everything else is the same.

wow much wow.

very review.

much good.

such work.

Solfn, You forget Skull + C4 isn't deal 40 xD

Fixed.

I wish the protective shield perk lasted longer, or at least the waiting time gets reduced to 15 or even 20. Just wasted 100 potatoes :c

^ You would be wasting 100 potatoes anyways if you were to buy any of these perks.

Awesome topic, very usefull !
I believe I'll use Shield and C4.

Since people keep asking about perks, i'm bumping this topic.

*bump* these are new and people need to know about them still XD

This topic should be added to the main post in the Fortoresse Q&A Thread because how perks work was not covered in the user-made guide.

Thank you so much, Solfn!

Anytime.
Some of the info I've noticed in other sources is somewhat inaccurate. Lots of people come around asking what perks do and I start to get annoyed when I have to keep redirecting them to the forum.