Fun to pick up, and even more so to get good at. This game will remain a Xbox Live Arcade classic for a very long time.
In Marble Blast Ultra you are a marble, trying to find gems (in multiplayer and most single player levels), and gain ranks & stuff. The game is fairly new so not too many people have bought it, I've only played one ranked match and it was a one on one, I lost...
However, when you first pick up this game it is amazing, but getting good at it is even cooler. It's sort of like a Sim style racer, when you have to take each turn perfectly to annihilate your aponents. Once you get a feel for the levels and how the marble moves, you can learn how to attack certain patterns in the track and totally obliterate you competition.
When I say track, I don't mean an actual race track, but the path, that always seems like a track, that everybody takes to get to the cluster of gems.
The single player is amazing, I've beaten to first two difficulties (easy & intermediate) but am having a hard time beating the last one. The dificulties aren't Halo-style when you just play the same level with harder oponents. Each difficulty has a set of tracks all its own.
Overall, this games amazing, the sound is good, though I recommend using custom soundtracks if you would like any sanity left after your first one hour sitting. The sound-effects are great, until you get the "Mega-Marble" power-up it feels like a real marble.
Great game, at least try the demo.