NOT overrated, don't believe the conspiracy theorists and hype-breakers that are trolling these boards! (updated 5/9/08)
No, GTA IV received a 10/10 from Gamespot because it's an amazing game. Not because it's perfect - no game can be perfect. Ocarina of Time received a 10/10 from gamespot, and that was back when gamespot wasn't using the 0.5 scoring increment system, but that game wasn't perfect for sure. Ocarina of Time was riddled with bugs, graphical hiccups, and other problems. But it was revolutionary for its time, and so is GTA IV.
GTA IV looks amazing, sounds amazing, and plays amazing. It places you in the shoes of Niko, an Eastern European immigrant (probably a Slovakian, but you never actually find out) who has landed in Liberty City, the virtual representation of New York City, who has come to America to live the big life with his cousin Roman, and to take revenge on a rat who sold his village back in [insert random Balkan country here]. Unfortunately, Niko gets screwed over multiple times in the first hours of the game, and has to fight his way to revenge and riches, overcoming some of the wierdest and most memorable NPC personalities you'll ever encounter in a video game. For example, you'll have to work for a crazy, juiced up "entrepreneur" called Brucie, who takes bull shark testosterone every hour and looks and sounds surprisingly like FPS Doug from Pure Pwnage. You'll cap some rival drug dealers for your Jamaican buddy Little Jacob, who may just have more Caribbean charm than Captin Pacard himself (if you've never been to YTMND, forget what I said).
The gameplay has been radically improved from previous GTA games. The shooting now consists of taking cover and using a combination of lock on and free aim to take out your opponents. In addition, your enemies will take realistic damage, limping if they get shot in the leg, dropping their guns when they get shot in the hands, and flailing on the ground in agony after you deplete their health bars. The combat system isn't perfect - removing lock on altogether probably would have been better for the series - but it's definitely gets the job done, and beats the pulp out of a lot of solely shooter games that have come out recently. The driving is less arcadey and more realistic, so you won't be able to round corners at 100 mph like in previous GTA games. Good if you're into immersion and realism, bad if you're an immature, impatient prick with no driving experience (see forums for details), and the cars all take realistic damage and can stall out if the engine gets too busted in. Although the game is a bit too linear for my tastes, there's still an amazing amount of things to do straight off the boat. You can play minigames such as bowling and pool (which could stand alone as games of their own), go out on dates, get drunk, steal cars for Brucie, shoot up gangsters for Little Jacob, take fares for Roman, and more. And the side quests are just as good as the main quest, so much so that you may get confused as to which quests you have to take and which you don't.
The game is shorter than you'd expect. Some reviewers have claimed the average playthrough for singleplayer is about 50 hours - I'd say that number is closer to 30 hours. I beat the main storyline and the majority of the side missions in just over 27 hours, not counting all the times I reloaded after going on rampages and failing missions, so expect to beat this game after a week or two of intensive gaming. As for the multiplayer... it sucks, don't bother. If your game isn't lagging and you aren't getting disconnected every two seconds, you'll be running halfway across the city just to be shot in the head by a guy using auto aim. Some modes, like cops and crooks and the coop missions, are moderately fun with friends, but they wear thin fast. The multiplayer was obviously just an afterthought, more of an experiment on the part of Rockstar than an actual commendable feature. You'll want to play this game for the singleplayer experience, which blows everything else in the action genre out of the water - while it lasts, at least. Maybe I'm just disappointed about the game's length because it was so damn amazing for the 30 hours it lasted. Get this game. Now.