Personally I found Saint's Row 2 better than this game, you will find at times boring, perhaps annoying...
The character-driven story is the best feature of this game, but apart from that nothing else is good, if you played Saint's Row, then perhaps you like to steal cars and keep them, to have a wide selection, even buy a house, i'm not talking about redecorating it, which is better than just buying it. But NO this game won't let you do anything like that, yes you can still steal cars, but then again you can only store one, you can't buy houses, they are unlocked according to your progress.
Only thing you can change is: your clothes. Plain simple, not much science, not much fun.
If we talk about story, definitely GTA IV is far better than it's predecessors, and you will be enthralled by it, the choice-making feature is a nice addition which we seldom see in this kind of games.
But after a few hours and missions, extra activities such as friendship grow really really tiresome, you'll get calls every 5 minutes from someone inviting you to do some activity, such as: eating, bowling, drinking, darts, etc. This particular part of the game is annoying beyond reason.
I say, rent it first, then if you like it buy it, but otherwise don't spend your money on this game, seriously it doesn't worth it.