First up, the guy below has got his reasoning wrong for the lack of planes. The REAL reason why there are no planes is because the game takes place in a city, and there is no reason at all for a city to have multiple airports. Hence the helicopters!
The game itself is brilliant fun, and is a departure from the previous GTAs. Your protagonist has come to America to experience the American dream, but it is not as he envisioned, having been given the wrong impression by his troubled cousin. But this storyline sets the way for great gameplay and graphics that are perhaps slightly dated now, but were great for their time.
The car handling is far more realistic this time, and you can see their suspension bob up and down when cornering, or driving on kerbs.
Along the way Niko can acquire several girlfriends, or meet up with friends for an evening out. To be honest this social aspect of Nikos life is probably the games flaw. It gets frustrating when you get constant phonecalls throughout the day asking if you fancy "doing something this evening", although initially it's a novelty worth exploring.
GTA 4 is gritty. Far more serious than the previous games - unless you play the "Ballad of Gay Tony" expansion, where the missions can be deemed, at best, as "somewhat unrealistic".
The Lost and Damned expansion is a bit boring to me. Biker gang warfare doesn't really interest me, and I hardly played this portion of the game.
Serious issues? Well, the frame-rate hiccups sometimes, and there are visual discrepancies, but overall GTA4 is a fantastic game. And, if you haven't got it already, this, along with the included expansions, is an absolute bargain at a penny shy of 18 pounds.