I rented it a few weeks ago. It's not that bad, the multiplayer mini games are kind of fun. It's not very challenging though, I beat the 20 race season in one sitting. You can unlock some different vehicles like an ambulance (complete with siren) and a firetruck. Sadly they do nothing for you but look neat.
There is a stat system where you earn stat points depending on how many cumulative points you get in your races. You can earn points by running over stuff on the sides of the track and doing stunts off of pink ramps. The points boost your speed, handling, etc.
The mini games are definitely the best part, especially Football (ie Soccer to us Americans). You and a friend drive a truck around a giant soccer field and try to push a huge soccer ball into the other goal. It's actually kind of tough and pretty fun. There are a couple others. One is combat where you kind of fight with your trucks. The other (I can't remember what it's called) is another where you try to push a ball around. There are a number of balls on each side and you try to push all of them to the opponents side.
I'd give the game a 6.5/10. It's fun for a while, the multiplayer is probably the best part, but the single player campaign leaves something to be desired. I think I definitely got my money's worth as a rental, but I'm glad I didn't buy it.