Instant classic, amazing game, epic title...all the cliches apply.
GTA IV represents a complete reworking of the Liberty City we all grew to love. While there are some similarities (the three-island system, names of certain locales, vehicles, etc., Lazlow...), the city itself has been completely rebuilt and expanded. The cast of characters is completely different (except for Lazlow...) and the story makes very little, if any, mention of the events of previous editions (except allusions to Lazlow's previous stops).
You'll adapt to life in Liberty alongside Niko Bellic, an Eastern-European immigrant with a checkered past. Tempted by his cousin Roman's stories of women and wealth, Niko disembarks from the merchant ship he works on to find out that things are not as he was led to believe.
Gameplay-The core mechanics remain the same. You explore the city by foot or vehicle, doing missions for various mob bosses and other ne'er-do-wells. But the way in which you do it has changed...for the better.
You can't talk about the control scheme in GTA IV without first mentioning the improved gunplay. Long a serious problem, Rockstar addressed it by enhancing the auto-lock with the ability to target specific areas of your enemy. If that doesn't work for you, you can pull the left trigger halfway and go into a free-aim mode.
Veterans of the series might be put off by the new driving mechanics. Instead of tearing around corners at top speed with the yank of the handbrake, cars and trucks actually react somewhat realistically (as if they had shocks). While the soft handing of the vehicles will be irritating to some, it works one you've become accustomed to it.
The story itself is a bit of a departure from the usual GTA formula. While the trademark dark humor remains intact (like Lazlow), the story is less one of "rags to riches" and more "struggling to get by".
Visuals-Grand Theft Auto has never been known as a graphic powerhouse. But the visual design is amazing this time around. The cityscapes are enormously detailed and, while there is still some draw-in, it's so far away that you won't really notice unless you're looking. It becomes obvious that upgraded visuals were a priority just by looking around at the sheer amount of stuff that is detailed in any given spot. Scuffed paint on apartment building doors, trashbag lined streets, and an amazing recreation of the Central Park and Broadway areas of New York put GTA on the short list of graphically impressive titles.
Sound-As usual, GTA's sound design is a home run. Excellent voice acting (without the usual celebrities, no less...except Lazlow) and a dizzying array of radio stations and music genres just continue the GTA legacy of standout sound. The ambient city sounds and explosions, vehicle noises, and other little things that you hear everyday in real life are all here. It's truly an aural tour de force.
Value-This is a no brainer. Rockstar always packs a staggering amount of stuff to do in their GTA titles, and this one's no different. While they've trimmed away a significant amount of fat between San Andreas and IV, the stuff that's left is excellent. The retooled Vigilante missions are a highlight, and the jettison-ing of the Ambulance and Fire missions is an addition by subtraction.
Oh, and by the way, they've finally added multiplayer. Whether you're looking for a deathmatch, a race, or a more focused mission (like Cops n' Crooks), if you're by yourself, in a team, or with one other person, there's something here for you. These missions are accessed through the single player campaign by way of your cellphone, and the array of modes and types to choose from is huge. As of now, most of the glitches that plagued early online play seem to have been fixed via a downloadable patch, so play should be smooth.
Once again, Rockstar has introduced a complete gem. This game has been a classic from the moment it hit store shelves. While the massive amount of changes made (except for Lazlow) for it's new-gen debut could have sunk the ship, GTA IV emerges with flying colors, paving a road that future titles can only improve upon.