Excellent driving simulation but poor racing simulation.
Gran Turismo 4 has amazing graphics, excellent sound effects and many hours of gameplay to be had. To hard-core driving game fans this is a wet-dream come true, but it is seriously lacking in the racing side of things: the opponent A.I. is practically non-existent, as all opponents follow a strict racing line, are completely unaware of your presence on the track and generally are just plain retarded. This leaves the game feel shallow, and un-challenging, like it was rushed to store shelves unfinished.
My recommendation is that if you’re looking for a hyper-realistic driving experience for your PS2, GT4 is the ticket. However, if you’re in the market for a nice racing game to keep you occupied for a good long time, this probably won’t do it for you.