Spectacular at first, not so spectacular after a while.
First of all I am not really a GTA fanatic. Other GTA which I have completed is GTA San Andreas.
Let's start at first with graphics. I am not the person who looks the best graphics, but they are pretty good. City looks alive and exploration worthy, especially at nights. Shooting and driving mechanics are two best updated things in GTA 4. Shooting in some tense places really resembles shootouts from movies like Heat etc. (even though later it kinda wears down).
Also interactions with pedestrians are pretty updated. You can push them aside, kick them out of way, and it's just funny to watch how the unlucky one is lying on the ground unconscious.
Noiw comes the main single player. Single Player missions start spectacular, with some excellet satire all over. Story is very promising at start, and you even start to hope that this time more single player missions will be more connected to main plot, but not, it get's into ridicilous amounts of sideplots, and even though outcome is kind of good, story at least 70% seemed like went to nowhere (altough as I said, it all wraps up nice).
And missions are more repetative than San Andreas. Sometimes you start to think that after a while they just pasted same thing over and over. And I say this honestly. Repetetive is the word of later missions. But even though repetetive, cutscenes are full of black humor, and Niko Bellic is a very interesting character, altough even a bit inconsistent, but good nonethless.
One other thing which is cool, is buddy system. You can go with buddies to drink, watch striptease, also girflfriend system is still there.
There are many other things (it doesn't pay me to tell all about them), but even though, you can clearly see the big cut of masiveness of San Andreas.
Overall: Overhyped is the word. Good game nonethless.