Excitebike + Modern Physics + Xbox360 rendering power = Fantastic.

They do just about everything right in this game.
-Tight responsive controls, easy to learn, hard to master
-Physics are seamlessly integrated and play to the core of the experience.
-Lots of variety in the levels, additional minigames, huge replayability. Well worth the $15.
-In game track editor with sharing of tracks (!) (Excitebike again)
-Great visuals with lots of special effects
-80s hair-metal music and 80s style screaming really help (for me at least!)
-Demos with playback
-Really slick way of comparing singleplayer races against your friends, integrated into singleplayer.

Check it out over at Xbox.com

The trailer does a pretty good job of showing the game off. Check it.