Could've been much better if the same amount of attention was put into the gameplay as there was with the aesthetics.
First off, I'll go with the good things. GTA IV has a good story; it's pretty much like playing through a gangster movie (the ending could've been better though...it kind of left the feeling of "that's it??"). Great voice-acting and facial expressions. The city is impressively big and detailed, and the weather and time of day even changes as you play! It's free-roaming. There's a cool cell phone that you can use that has been implemented really well and there are plenty of mini-games that you can play to maximize the time you play the game. You can even watch a few TV shows, browse the game's internet, go to comedy clubs and even a strip bar and get a private strip dance! And you begin to think, man, is there anything this game doesn't do?! But don't let all this aesthetic detail cloud your judgement of the gameplay. There are many technical problems that GTA presents. GTA IV is probably one of the most frustrating games I've played in a while, and it's challenging not so much in the satisfying sense, but in the frustrating sense that the mechanics are just not that great.
The vehicle handling is so freakin loose that it's really hard to control and this is incredibly detrimental to car chases, losing the cops and even just driving around to get from point A to B! To make matters worse, some car chases involve shooting the other car down until it explodes. With all the traffic, the loose handling and trying to aim as well, it's almost impossible! If you get in a bad crash once, it's pretty much over and you have to do the WHOLE THING over again, which means you have to take the time to get to your location first, regain health if you've lost any, etc. In short, driving can become tedious and quite frustrating.
The shooting presents problems as well. A cover system has been implemented in this game, but it doesn't work very well and is inconsistent. Many times you'll stick to the wrong wall or not stick at all! When shooting from cover, you move SOOO SLOW to shoot. There's also an auto lock-on feature to lock-on to baddies, but many times you'll lock onto to the wrong guy or you'll have to un-lock and change your body's direction if you want to lock-on to another guy. Another issue is that you DON'T regain health if you stay in cover that so many other present shooting games do. Because of that, the speed at which you can die can be very frustrating (and of course, once you die, you'll have to re-collect armor, ammo, drive back to the location...UGH!!!) Fortunately, much of the AI is stupid enough to just stay in one place so you can pick them off without moving around too much. And there's plenty of money that you receive from missions and ammo as well so that's never really a problem.
Now, it is possible to work with these issues and actually progress through the game, but DAMN it can be really frustrating at times! The missions aren't impossible, but many of them end up being a tedious and frustrating trial & error formula. This game could've been so much better if the same amount of attention was put into the gameplay as there was with the aesthetic detail of the game. But now that I think about it, even the aesthetics in the graphics sense aren't that great. The graphics are more like really good PS2 graphics rather than PS3!
But despite all those complaints, GTA IV is still fun once you get the hang of it and is definitely worth playing through. Just be ready to get frustrated at times!