I had played this game a few times in the past, but waited until the price dropped to something (IMHO) reasonable. Since I am not an avid gamer, I really just wander around the GTA world, Liberty City, stealing cars and generally causing trouble. There are plenty of "missions" to interest those that want to rack up achievements and properly progress through the game. Personally, I would rather the whole thing was just opened up without restrictions so I could just explore. But that's because I never spend more than about ten or fifteen minutes playing the game and have no real intention of getting serious about it.
So, probably not the best game review ever. I guess I am writing this more for the casual gamer. In that sense, GTA IV is simply a fun distraction and an opportunity to act out and misbehave vicariously through the on-screen character.