I've had GTA IV on the PC for a while (already had it on 360, but it was cheap to re-buy on the PC and I wanted to try some mods and stuff)
I don't recall ever running into any major bugs, at least not any more bugs than in the 360 version. It's kind of messed up how much BS you have to wade through to just get into the game and start playing though. Your Steam account apparently isn't enough. You have to set up a Games for Windows profile (which you can connect to your XBox live account if you have one) in order to be able to save your games. And if you want to play multiplayer, then you ALSO need to sign up for a "Social Club" account. Frickin' three accounts to play one game, it's ridiculous.