I find it entertaining. Although better graphics and environment interaction would be great, I never really think about it. Half the time the scenery is just a blur from the insane speed.
Personally, I only wish they had more multiplayer capabilities, either for 2 player or online 4 player. To me, that's what replay value is all about. Hell, Mario Kart can be beaten in 1 easy sitting, but the replay value is all in the multiplayer.