The game is definatley worth it, I heard the PS2 version was the best one and had the best effects. The game does have some faults such as you losing all your upgrades when you get a new car, but the amount of new features is very big and makes up for the faults. Also, you can cruise around freely in the city now and you get to your races, etc by driving to them.
New features are driving around the city, removal of the "knockout" mode and instaed there are some such as the Street X races and the URL (Underground Racing League). There are some great new cars (Cadillac Escalade, Hummer H2
, Lincoln Navigator, etc), and the new customizations are huge. They include spinners, hydraluics (?sp), audio systems, changing small details such as side mirrors and more.
Bottom line - I definatley recommend it.
"The greatest threat to knowledge is not ignorance, it is the illusion of knowledge."