Decent, but shallow game, and not worth 800 points
There are three sets of difficulty levels, Beginner, Intermediate, and Advanced, each with a set of maps. Most players can beat beginner and intermediate under par, but advanced is quite difficult and challenging. Maps may have obstacles, elevators, gravity-direction changers, iced-floors, bumpers, and other things that challenge you to get through them.
It starts out fun as you try to beat the levels, but once you get through most of them and under par, there's no incentive to play them again. Multiplayer is okay: you try to collect the most gems and points to win. But other than that, the game loses replay value, since even that gets monotonous.
Graphics are good, sound is good, but music gets annoying, so I recommend you listen with your own soundtrack.
For the short and shallow game it is, Marble Blast Ultra is not worth 800 marketplace points. While fun to go through and beat the maps, there's not much else to do when you're done.