I own it for my PC, bought it at gamespot and downloaded it. No problems, no hassles, and looks great. My PC is going on 2 years old now and has no problems running it with nearly every setting maxed out. Have the computer plugged into the reciever, so I get the good experience out of it.
My buddy has the Xbox version of the game, it too is also great but I'm an old timer when it comes to playing games via PC, and therefore prefer to play it there. My issue with the Xbox version is the lack of customability for online play. With the PC version, you go into multiplayer and you can take a look at the server list. Don't like the Kill Cam? With the PC version simply filter it out. Want a server with more then just 16 players? fine, find one with 50. Want a dedicated server that doesn't force you to find other users constantly, too easy.
The only real benefit I see to the Xbox/PS3 versions is that you can get whats known as prestige mode. Once you hit level 55 you can trade in your rank, and get a fancy symbol by your name, and you get to work back up from lvl 1 again. You can do this up to 10 times in the PS3 and Xbox versions, so thats technically a rank scale of 550 ranks... Not bad.
Some further thoughts about CoD4. Well, I believe that CoD4 did a wonderful job on it, but there really is so much more that could have been done with it. Take a look at PC games like BF2 for instance. Modern warfare includes the usage of vehicles, I should know, I'm in the Infantry. Games like Halo also have been using vehicles for quite some time now, so why not include these components to expand upon an already good game? How does a 3 kill streak, or a 5 kill streak, or a 7 kill streak simulate the reality of combat at all ? At 3 kills you can call in Radar, at 5 you can call in an Air Strike, and at 7 you can call in a gunship for fire support. Looking at a game like BF2 for instance, you have a more realistic balance. You have an overall commander, and then section leaders who can request artillery. CoD4 could have done something similar relatively easily.
The rank system in CoD4 is good, but simplistic, again comparing it to BF2 you gain levels, and as you gain levels you unlock weapons. CoD4's rank system offers some unique things that I think are definently better then BF2's for instance, the ideas of unlocking challenges that once completed help out a lot with your drive for experience. However, it falls short in a few things where a older game like BF2 surpasses it. You don't get to enjoy the leveling up experience as much because you don't quite get the same sort of knick knacks that you do in BF2. Campaign stars, ribbons, badges for excellence in certain categories. Nor does CoD4 track all of your multiplayer stats as well. I'm curious how many rounds I've shot on the M4, what percentage were missed, and what was head shots. I'm curious about my average lifespan, and the ratio of kills to deaths.
All in all though, its a fun game, and you wouldn't be disappointed with buying it. As to the realism of it, well, its a game. It doesn't get anywhere near close to the real thing, and that I can assure you from having been there myself. My suggestion for someone wanting to experience the real thing would be to anti-up and join the military, otherwise your going to continue to believe these games are realistic, which they are no where near it.
Airborne!