I have several builds that I play often, mostly swapping because I find myself having trouble with a build on a given map.
I have 1 knifing build - it's got Marathon Pro, Lightfoot Pro, Commando Pro (this one is my favorite, not for the extended knife range, but the fall without damage is pretty awesome)
I have a few stealth builds (mostly for different weapons) that are either Bling Pro or Slight of Hand Pro, Cold Blooded (I'm pretty sure I have this one Pro too), Ninja Pro
Then I have a few straggler builds that use Stopping power or Hardline for my second perk and steady aim pro on the third.
I have trouble with a lot of guns tho - many people seem to be able to kill me with one or two shots from the guns I use but it takes me quite a few more rounds to kill someone - really rather annoying.
Of course, also, on the PC world, the Hacks are terrible (aimbots, wall hacks) that doesn't seem to be as much of a problem for the PS3/360 version. Worst thing IW did was take away dedicated servers for us PC people, basically took away our one tool for dealing with cheaters when the anti hack didn't work (anti hack on MW2 being Valve Anti Cheat which sucks) Plus it's kinda annoying HAVING to have an internet connection when you're playing with friends on a lan for a lan party. Really restricts where one can have a lan party to somewhere with a decent net connection that is willing to share.