Treyarch just released the long-awaited patch for PC.
It pretty much fixed many of my issues, but I still encountered a random freeze when I was running through Vietnam's jungles. It made me a bit sad since I hadn't saved/quit the game yet. Apparently, quick save (F5) doesn't work on my installation. I actually picked the second-hardest difficulty which didn't make it better when I was trying to survive with lag.

I actually managed to have a decent playing session despite my issues although they weren't so much stutter as occasional lag, especially in Vietnam when you have to fight your way down a hill, and throw barrels at them.
Anyway, great game they've made, I can finally start to appreciate the amount of work they've put into this game.
I do hope Infinity Ward gets into the game again. I'd be very much interested in seeing a Modern Warfare 3.