Just plain fun.
Heck, you can't even hit the other cars.
However, the game is nicely done and there is a thriving community that keeps posting new tracks and other great stuff.
If you are new to the trackmania series, than you must know that the main challenge here is to beat the clock . (Although the platform and puzzle modes are also interesting).
By downloading and patching through United Forever you can access all the user made creations and new stunt mode.
Long time players may recall the Stunt Car Driver in the Commodore Amiga, that mixed rollercoaster with a driving game with incredible success.
The same applies here, it´s just plain fun and well executed.
Graphics are good and sound is ok. The ladder system in the multiplayer is very well done, so that each week you have new tracks and files to download.