SSX on Tour is simply amazing. And while it's not a perfect game, it really shines as one of the PSP's best games. If you aren't familiar with the SSX series, the games are about skiing and snowboarding, and competing in different challenges and races to gain fame and fortune.
In this game, as the title implies, you go 'on tour' and compete to be the best snowboarder/skiier around. The gameplay is amazing, but I enjoyed the races moreso than the trick competitions. The controls are easy to get into, but they aren't perfect for doing complex tricks and such. The graphics are among the PSP's best. Beautiful lighting effects, advanced shadows, and wonderful character models take this game to the top.
Overall, if you can pick this up cheap, this is a no-brainer. If you would rather get a console version of the game, go for it, but I actually had more fun with the PSP version. Either way, this game is a definite purchase for any snow-loving gamers.