For $15, no game gives you more for your money than Trials Evolution.
Everything that was fun from the first game is still here. The same gameplay still exists, getting from start to finish through obstacles that range from easy to hard to mind-numbing insane. There are very few changes to the actual gameplay mechanics that made Trials HD work so well, but the main changes come in the form of the environments. Instead of being inside of a warehouse, tracks now take place outdoors, and while this doesn't seem like that much of a difference, but they do give the game I nice visual improvement. It also makes a big difference on the track design, which now includes curved driving lines, so now tracks can be much more diverse. The level design in the game is fantastic, from the easy tracks with big jumps, to the LIMBO-inspired track, to the controller-chucking difficult Inferno 3. There are tracks in this game that everyone can enjoy, no matter what the skill level.
There are three main additions to Evolution, the first of which is multiplayer. Online, there are 3 modes to choose, two of which are just races on Trials tracks. The third one, Supercross, is the only mode that can be played both online and offline. I've yet to play it offline, but I found it an interesting way to support multiplayer, and it is pretty fun. It seems like something that would be enjoyed if you and a bunch of your friends were just hanging around (unless, of course, one of you is much better than the rest).
Second, there is the track editor, which is insane. The amount of tools available to the player to edit their own track is beyond count, and can support almost any game design possible. Plus, as shown by the skill games, there is a lot more to create than just Trials tracks. Almost any game can be created. Some of the examples given by RedLynx were Foosball and an FPS. Given time, there will be plenty of great user-created tracks to download.
Which brings to the final, and most important, addition to Evolution, the track sharing tools, which is the one thing that really separates Evolution from HD. Those who played HD know that the only way to share tracks was to have people on your friends list. RedLynx realized how terrible this was and decided to make it better. So, with all of the editing tools available to the player, and the much improved sharing system, the number of tracks available to you is limitless.
This game isn't without flaws, of course. The are some technical hiccups when you reload a checkpoint that is really far away and the textures have to take some time to load. Some of the skill games aren't interesting, and some are just flat out terrible. Plus, the customization options for your rider aren't very interesting.
But for $15, there really shouldn't be anything that stops you from buying this game. With great design, fun gameplay and an unlimited number of tracks at your disposal, Trials Evolution is one of the best XBLA games of all time.