Like so many people have said before once you finish the game, it can get boring. But there are plenty of ways and things you can do to make it last longer and stay fun.
You get out what you put in!
I've been playing TestDriveUnlimited on the 360 since January 07, finished it in April 07, and since then have put over 600 hrs playtime on it without restarting. That game has less to do than GTAIV also.