I pre-ordered, and I bought a copy for my friend today.
It's currently breaking records on Steam right now, with most concurrent users playing in one day. Nearly 250,000 people. MW3 has about 70k/80k (Single-player and Multi-player respectively) and that's saying something.
I haven't been able to pull up past stats, but a quarter million playing at the same time is a lot of people for PC. A lot of Steam veterans are saying it's the most they've ever seen for one game, not sure if I believe it, but I wouldn't doubt it either.
Lots of people playing this game on PC. I played 16hrs since midnight release. Totally addicted.
(Also, it's not even fully released yet, as this thread attests to. Not all regions have been unlocked/released.)