Customization and a solid online brings this one as an excellent game.
Online was great and I had friends from elementary school play this game with me. It was fun from driving a Hummer with Underglow neons, to a Subaru Impreza with a HUGE spoiler to show off to your buddies.
The single player itself is very good. The story is lacking but it is a very small part of the game to just explain why you are racing in the first place. Easily, it would take tens of hours to finish the game but it kept you busy.
On the technical side, graphics were pretty good at the time, but not amazing. However, I think the graphics excel artistically. Because of it's "California" feel and vivid colors all around the city. The sound is another great thing. The engine note is very accurate with stock sounds and probably the best I heard back then (probably EVEN NOW)!
Now the customization... easily one of the best if not the best. The only game that could compete is Midnight Club but you couldn't really visually customize all cars equally in Midnight Club 3 anyways. Here is a list of all the things you could customize:
-Roof scoop paint
-Side mirror paint
-Brake Calipers paint
-Engine Accent paint
-Muffler Tip paint
THAT'S ALL FOLKS! AN INSTANT CLASSIC!