It's definitely the best Grand Theft Auto game, but it does have its flaws.
One of the major flaws, albeit somewhat minor, is that the game is a bit too much like real life, in terms of your relationships and such. You can hang out with friends and date women, but once you do, you have to maintain those relationships by doing activities with them. Many characters have special abilities, and in order to keep those abilities available to you, you have to maintain those relationships. These relationships also interfere with missions on occasion: you will be driving to a location and a friend will call you asking to go bowling/eating/drinking, and you have to decline and lose some of their friendship as a result of that. It's definitely minor, but I think it could have been implemented differently.
The second and last flaw that I can think of is the implementation of the cover system. It's a Gears of War-type of system, but it can sometimes be annoying because Niko will get behind the wrong cover when there are multiple objects to hide behind, and Niko will get hurt badly as a result. There were a few times when I would try to get behind cover, but it was the wrong cover, and I would die as a result. Other than that, the cover system works very well.
Now that the bad things are out of the way: GTA IV is an amazing game. The city itself is extremely immersive, especially when flying around in a helicopter. Flying high above Liberty City is a treat that every gamer should experience, because it's so intricate, and it looks so much like New York. The minigames that you can do with friends are very fun as well, like bowling, darts, pool, and drinking. Going drinking is pretty funny, especially with a girlfriend, because she will get just as hammered as you are and will stumble around, saying things that don't make sense. The cars are amazing to drive, especially the ones that handle turns very well, such as the Turismo and Coquette. The story is kind of confusing, but only because the very nature of the game causes you to forget what's going on as you do other missions or hang out with friends.
When all is said and done, GTA IV is a game that was well worth waiting for, and is definitely worth the $59.99 price tag.