Great even when you beat it!
Also, the new 2 player mode where you can roam the city and kill people is also a great add on to the GTA series. I have always wanted a 2 player GTA and this is thier first step in that process (If you also like 2 player GTA style gameplay, try out The Warriors). The second player gets to decide what his character looks like out of the civilians that live in that area. Also, 1st and 2nd player can kiss whenever they are close to eachother which is very amusing when you are messing around. Even if you dont have anyone to play with you can recruit people from your gang to come into your car and take over some enemy territory. Eventually, you can take over all of the territory.
Another great add on was the skills that your character could get. Eventually, your aim gets better, you can duel wield, and drive better. You can go to the GYM and work out your character to make him stronger and less fat. You can go swimming and be able to breath under water for longer. As you advance to different cities, you can learn different fighting movese (Kick Boxing, Kung Fu, Etc). Best of all, you can ride bikes incredibly fast and jump extremeely high (almost unrealisticaly) over other cars. Also, the customization of your character gave you many options. You could change your costume, change your hair, add tatoos, add "bling", anything that you want.
Not only is the gameplay extremely great, but even after you beat the game there is still a lot that you can do. For example, there are quite a few mini games to play like side scrollers. Also, with the many cheat codes that completely change the physics you can mess around in a flying car or not mess with the physics and just flay around in your very own jetpack.
Even if you havent liked the other GTA's, I definately recommend this GTA. This IMO is the best GTA game, and I cant wait till the next one, who knows what they will add next.