Though GTA IV is certainly fun, it lacks all of the little things that made the previous entries to fun and long-lived.
...and I'm not talking about airplanes. No, I'm talking about those little goals, those distracting side-angles, the rewarding exploration, the purpose of money, those small little details that really fleshed out and sold the previous GTA games.
Sure, this one is fun, and certainly it has the best story of the lot...but that's about it. There's nothing rewarding about it at all, other than playing through the story. Once that's done, it's all done pretty much. There's a few side things to do, a few assassination missions, car-jackings, and other small odd jobs...but it's all negligible really, when it's all added up.
GTA IV is a single-minded affair, and this is why it fails, compared to its previous entities. The story and engine are much better, the world is a bit better realized, but the content is just not quite there.
I'm not going to rant, but rather, I'm going to sum this up pretty quickly and put it under wraps:
GTA IV delivers in: it's story, the environment, the characters, the side-goals it DOES have are pretty fun
GTA IV doesn't deliver in: its treatment of the economy, its treatment of friends, its soundtrack (beside a few nice tracks here and there), its lack of things to do out of the almost exclusive main storyline, its lack of reward for exploration, its lack of the smaller, finer things that made the previous entities so much fun to keep coming back to, its overall lack of fun distractions
Sure, it's still fun, worth a pick-up (although I honestly wish I had rented it first), and certainly worth a playthrough, but until some sizable expansions come out (DLC), it's not worth much else.