Each week there's one track to compete on, and you can't use all the tracks, just that one. The courses are fairly long, and really fast. Control is just via the cursor keys, making it really easy to pick up, but finding the fastest line and not crashing is harder.
You can practice and qualify as many times as you like, once you've registered. When qualifying, you "race" a ghost, which you can set. By default it will be the next fastest competitor to you, based on your training time. You can race any ghost in the leaderboard though, so choosing the fastest rider means you can learn their tricks.
MountainBike Challenge looks good, with long views down the mountains as you ride, and the sound effects are good enough. The downside of this is that there is only one track to play at a time, and while it's easy to play, it's a shame you can't lean back and forward on your bike.
The online element makes this simple game competitive. The sense of speed is good, but overall it's a bit limited.