Tons of hours and it never gets boring!!!
First off, it has a great storyline. I love how they incorporated the idea of gang territories and a girlffriend. Also, the idea of excercising and eating also was a great addition. This only made the game more realistic.
Second, the missions are awesome. I didnt think that Vice City was able to be topped, but I was wrong! And thank goodness! The side missions, of course, are also fun. The races and other mini-games just added to the length of the game and the longevity of time that we will all play.
Third, the graphics were a pretty good step-up and the addition of new, sweet cars was super cool.
Fourth, greeeaaat soundtrack. Couldnt get enough of it.