Bill Harris played five hours of GTA IV then posted his initial impressions. He's got a lot to say about the game so far, but this summarizes his thoughts:
There are so many clunky moments like that where something should be fun and immersive, but instead it's a reminder of just how thin the world really is, and it's frustrating.
That describes the game in a nutshell. It's vast, and detailed, and paper-thin. Put your hand on that paper and push, even gently, and your hand goes right through.
Note that he's referring to the X-Box version of the game. You can read the rest here.