It's not 'cause I'm a fanboy
Simply put, this is a very fun game. Yes, it recycles the tracks, music, and menus from Gran Turismo 4, but the racing here is very different. One thing that I tended to lose appreciation for while playing Gran Turismo 4 was the great vehicle physics. Reason being is that I could simply take most of my cars and upgrade them to a point where poor racing technique could be overshadowed power, suspension, exhaust, weight reduction gadgetry, and other myriad off-the-track techniques.
Not having the said vehicle enhancement options in Tourist Trophy is this game's double-edged sword. While you can't save money to purchase parts and upgrades, you can appreciate the awesome motorcycle physics in this game. They make racing very fun, and very unforgiving, and thus even more fun.
Unless you have previous motorcycle experience, you will be the woman/man if you can make the first turn without turning over your bike and sliding across the track. After getting the hang of motorcycle racing, you really start to appreciate the sense of speed and grit-your-teeth adrenaline that this game evokes. The graphics are amazing, and the behind-the-handle-bars view is implemented extremely well. It is obvious that the developers spent a lot of time on the first person feature, which really pays off for the gamer. The bikes are extremely detailed (after playing this I would like to own a crotch rocket) and the replays are satisfying to watch. Here you will see your rider tuck, bob, gas, and shift gears on the bike in an extremely realistic manner.
In order to unlock bikes to be used in competitions, they have to be won in challenge mode. Here you race on the bike that you are trying to unlock against an opponent on either an identical bike, or similarly spec'd bike. This happens to be the only mode I've been playing for the past few days because it is very fun unlocking the bikes. These bikes go to your garage where you can select them for different races in the game's championship mode. They look awesome and each handle with their own idiosynchrosies.
The last motorcycle game I played was Excitebike 64, so my frame of reference might not be the best. However, in my opinion, if you are looking for a very fun/realisitc/satisfying motorcycle racing experience, then give this game a whirl.