Um... wheres the shop?
To me the challenge races basically feel like license exams. Meaning you will fail over and over again, playing the same track over and over again. I found this process very boring after the first few hours. I would rather race for credits and work towards buying a really cool bike, or winning one from a race.
While, we're on the subject of races, the bikes you win from normal races pretty much all suck, or are no faster than the the bike you used to win that race. So really what is the incentive to win races? If you cant buy or sell bikes, and the best bikes can be won through challenges, what is the incentive to win races? I need incentives to spend the time beating the races, and this game does not offer good incentives.
On the plus side, the racing itself is quite good and there are a lot bikes to win and tracks learn. This is definitely not "Gran Turismo with bikes"; instead its a totally unique game and the bikes handle nothing like the cars from GT. But without a shop system (cant buy bikes, sell bikes, or buy upgrades) to me the game feels gutted.
The game is now widely available for under $20. For that price it offers a very good value, if you can stand the frustration of failing over and over again. Casual gamers should probably avoid this game.