So what I will be doing is give a quick rough review about the perks on Modern Warfare 2 and how they can benefit the player. Note: Some of this information may be inaccurate.
Perk 1
Marathon [DEFAULT] - Marathon is usefull if you are capable of 'rushing' as people would call it. Its VERY usefull when you intend to run & knife, with the addition of lightweight. Marathon gives you unlimited sprint therefore your character will remain at a very fast pace. When using marathon pro it allows you to climb obstacles faster. I don't think theres much of a difference but I suppose faster climbing will help on a map like Favela.
Sleight of Hand [DEFAULT] - Sleight of hand pro is a very useful perk for those Light machine users, it will double the speed of your average reloading time making it very easy to run & gun with LMG's. It is also a very usefull perk when it comes to run & gunning, when you run and gun you find yourself constantly on the move, always shooting. Reloading at a low speed can cause deaths which I'm sure we're mostly aware of. Sleight of hand pro gives faster aiming, this means you scope in faster, I really don't think its a very good addition but when it comes to running and gunning again it comes in handy
Scavenger [RANK 13] - Scavenger is a new favourite of mine, basically when you run over a body with a blue bag you will pick up ammunition, equipment and special grenades (if they have been used already). I find this to be a must-have as when I'm playing I can't afford to run out of ammunition or grenades, I also like to have a back-up. Its very necessary on games like: Domination, Headquaters, Capture the flag. Games where you are normally alive for long periods of time, that is how on Search and Destroy I wouldn't recommend it, the rounds last about 3 minutes and you are alive for normally 2 minutes per round on average, plus on search and destroy they amount of dead bodies would be increasingly low. Scavenger pro allows your primary and secondary guns to have extra mags, which gives your guns, I think, 100 more bullets. That is A LOT when it comes to being on your feet in matches.
Bling [RANK 21] - Bling is a really good perk to have it allows your primary weapon to have 2 attachments, e.g. an AK-47 with red dot sight and grenade launchers. This perk is much more useful if you choose the attachments: (...............) & silencer. In my opinion, silencers are a very very good attachment to have on a gun, it will keep you hidden on the radar and you can still have good accuracy if you add a red dot sight too. This perk is often used on Search and Destroy as people prefer to remain disguised, but have a good chance of getting a kill. Bling pro allows your secondary weapon to have 2 attachments, so say, you've lost all ammunition in your primary weapon, I'm sure a PP2000 with a silencer and holographic sight can be just as good.
One Man Army [RANK 45] - This perk I've not used for a long period of time, but in the time I have used it I've found a lot of good things about it. Well I'll begin with its probably ONLY downside, it replaces your secondary weapon slot, however, it shouldn't make a difference because if you feel you're running low on ammunition one man army will basically reset the class and give you your equipment and special grenade(s)'s back. I found this useful when I used LMG's and had to reload, one man army just took about 5 seconds to give me the original class back. One man army pro will allow you to swap your classes faster which can come in handy if your in a crowded area and want to get some kills fast.
Perk 2
Stopping Power [DEFAULT] - Stopping power as I know will be a favourite by most of you, it increases bullet damage on your weapons making kills faster and easier. Stopping Power has always been a perk I've used lots but like I said, I like to very my perks. Stopping power is a perk which will always be used by the majority of Call of Duty, but it can be beaten just by good aiming and accuracy. Stopping power pro gives extra damage to vehicles, so when shooting down the enemies killstreaks it helps alot, I do recommend it but they're are some other usefull perks to use instead.
Lightweight [DEFAULT] - Lightweight allows you to sprint twice as fast, this perk is a great combination with marathon. The reason these two perks work so well is because you can run so fast and if you have quick reactions you can be a very hard-to-kill opponent. I highly recommend if using lightweight you add marathon as your Perk 1. Lightweight pro will allow you to move quicker whilst aiming, therefore drop shotting, sniping, run & gun becomes easier. Lightweight is a good perk but using with the correct perk 1 and perk 3 it can be VERY good, which I will come onto later.
Hardline [RANK 9] - Hardline "The perk for noobs" well, no actually, its a perk which a lot of 'pro' people will be found using, hardline makes your killstreaks 1 less kill to get so you can have a Harrier at a 6 killstreak (highly recommended 1st killstreak when using Hardline). It is said that noobs use hardline to get their killstreaks quicker, well why not? 1 less kill can help a lot more than you think, getting nukes are far more easy too. Hardline pro allows your death streak to kick-in 1 death earlier too, I don't see how that is a helpful thing unless you are in a game of being repeatability spawn-killed, but oh well.
Cold Blooded [RANK 25] - Cold Blodded is usefull, but not on all maps. Maps like Wasteland, Underpass, dark and rainy maps, where thermal snipers will be on-the-go. Instead of appearing as a visible person on the thermal scope, you sort of blend in with your surroundings. Cold blooded pro is usefull on every map type, if the enemy is to put there crosshairs over your body, they wont turn red, the crosshairs will remain white - as if nobody is there. Also, your username wont appear above your character, which can sometimes be a big give away.
Danger Close [RANK 33] - Danger Close is a perk for all the noob-tubers, it increases explosive damage by multiplying it by 1.4 therefore it can kill enemies where as sometimes you may get hitmarkers. It works the same with frag grenades, semtex's, claymores etc.. Every explosive. Danger close pro increases explosive damage from all killstreaks, in my opinion it is not very usefll unless I intend to continue noob tubbing, I'd normally just shoot the two then use the gun normally.
Perk 3
Steady Aim [DEFAULT] - Steady Aim is the type of perk you'd use for a gun with lots of recoil, e.g. MP5K. If you are an experienced Call of Duty player you should have no trouble aiming and being accurate anyway. I think it does make a difference when you first use guns like the P90, but when you feel confident enough to start using stuff like Last Chance, go for it. Steady aim pro lets you take longer breathes in while sniping, therefore whilst lining up the perfect shot you don't have to rush yourself, just take it easy and try and get a 1 shot 1 kill.
Commando [DEFAULT] - Commando is a very useful perk to have on your class, it will allow your character to improve their melee distance, so that when you are about to knife somebody you may take an extra foot forward and get the kill, it is annoying if you are on he receiving end of the knife but it is a very good perk. Cammando is particularly good when you have you use marathon and lightweight the reason these perks work so well together is because your player can be so fast that your opponent may not even be able to kill you as you run towards them, and with the advantage of extra melee distance, you will more the likely kill your opponent using a knife. Commando pro reduces fall damage, I find this very good because I tend to jump out buildings alot and when you do that without commando pro you will injure yourself making you very weak, with the perk you can just jump out and not feel any damage and continue your game.
Scrambler [RANK 17] - Scrambler is a perk which I use to think was great, scrambler is a perk in which if you are approaching an enemy they're radar will slowly become faulty until you are dead/far away. The reason I've avoided scrambler is because well, when the enemy's radar is faulty they know that you are near them and will begin to look for an opponent. The Pro version of this perk protects players from enemy Claymores by increasing the time between detection and detonation.
Ninja [RANK 29] - Ninja is a perk which a lot of people will use on search and destroy as you wont be picked up on heartbeat sensors, it comes in handy a lot when trying to sneak around big maps without getting caught up by campers who are expecting you to walk past. Ninja pro allows your character to move around with silent footsteps, this is another reason to use this perk on search and destroy, especially when you are the last man alive and need to plant the bomb quickly.
Sitrep [RANK 37] - Sitrep is what most of us will remember as "Bomb Squad" where you can be aware of enemy explosives, I personally don't use the perk due to the fact that I don't think a lot of players use claymores very often and I think that there are better perks to use instead. Sitrep pro was a perk I used on almost every class on World at War, but that was because almost everybody on search and destroy would use bouncing betties. Sitrep pro however, makes the enemy footsteps louder which will come in handy on lots of game modes, whether your camping on demolition, waiting for the bomb carrier on search and destroy, anything. I do however feel it isn't as good as say, commando, but a lot of people will disagree with me, but its just my opinion.
Last Stand [RANK 41] - Last but not least, last stand, a VERY good perk to use, probably the best perk 3 to put on your class. When somebody shoots enough bullets to kill you, you will be put on last stand which is basically lieing on the ground with your secondary weapon trying to kill your opponent. It is good on fast moving games like demolition, capture the flag etc... Last stand pro will allow you to use equipment on your last stand, it is fairly better but I tend to just keep shooting if I see anybody, although random grenade kills FTW.
I hope you have found a good perk 1, perk 2, perk 3 to use on your classes and I hope you have realised how usefull some of the other perks can be. Thanks and please leave a reply to this topic with your opinions and your favourite setup and why you use it.