The best GTA game so far. Needs a pretty good PC to run smoothly. A lot of stuff to do and the plot isn't that bad either. If you like sandbox games you will probably like this one too. There are few annoying things (controlling helicopters, some random deaths..) but otherwise the game is awesome!
Oh and Episodes from Liberty City is pretty good too though there isn't as much stuff to do as in the maingame. The Ballad of Gay Tony is better than The Lost and Damned but it's not that bad.
If you like doing every possible thing what you can in your games like completing all the mission even after the main ones are done, there will be a lot of stuff for you in GTA IV.
Oh yeah.. there is a multiplayer mode too. Haven't played it that much but it seems to be okay. There are a few co-op missions too but haven't tried em cos no one seems to play em online.
5 out of 5! or 10/10! or.. 7½/7½!!