Well, where to start...
I've been a fan of this series since the very first installment, I'm a big fan of a lot of different genres - even Indie games like Mount and Blade. As such, I'm quite used to bugs and figuring out how to work around them. GTA IV is however one of the worst experiences I've had when it comes to installing, fixing and trying to get the software to work. Yes, you've probably read/heard that from many others, but it's true...
After 2 days I've been able to get about 8 hours out of it, the game crashes way too often (every 10 minutes or so on my system), I experience weird bugs that seem to be quite random. I fully meet recommended requirements, so the problem is most likely optimization. Rockstar Games simply didn't test their game properly.
If you spend some time on forums with sections dedicated to fixing the game, you'll see the vast ocean of complaints and raging fans who swear "I'll never buy anything from Rockstar again!!!1" - this is of course an overreaction, but it's still symptomatic to the inherit problems of this game.
I'm not trying to say that Rockstar is the devil incarnated, but they really dropped the ball on this release and it'll cost them a great deal of respect. I find it even harder to understand why such a solid company would be willing to print it's name on a such a release..
Bottom line: by Q1 of 2009 this game will probably have been fixed with a bucket-load of patches, I highly recommend waiting until then! You'll most likely be frustrated, like so many others, if you jump on the wagon now.
- Edit update -
It's now march and the game is still unplayable for me, bugs and CTDs are common and every patch has only introduced more bugs to my game. This is probably the worst experience I've ever had in gaming.. and I've had some pretty bad ones! "thank you" Rockstar for setting a new standard..