it gets even better and better, try for yourself!
For myself I think thats the best is the Racing Career, first you get some tutorial, than lets start the grandprix, first, there are 3 levels to choice from-rookie, pro and champion, after you beat champion, you get to choice the legend level, thats when the fun really starts ( see for yourself:) ).
after some racing practice you can try some challenges in your career, thats will be awarded with medals(bronze, silver, gold) for the gold you get also one experience point. the grandprix is great, but my self i like mostly the older Extreme class, this time no more 600,900,1200 classes, just one. a lot of bikes to choice from, well, this time you must work harder to get the money needed to buy and upgrade the bikes. you can get realy lots of upgrades for the bikes: tires, wheels, engine, brakes, suspension, swingarm, exhaust, bodywork. sounds great:)
So try this game, it's worth the money