It's much more fun that it looks.
The difficulty level is varied because there are some levels that are so easy it seems like a 3 year old could beat it. However, there are some levels that are so impossibly hard that it makes you wonder if it is even beatable. There are some levels where I just gawk at how hard it is and how anybody can finish the level. With over 60 levels of action-packed goodness, Marble Blast Ultra will keep you going for weeks.
The Multiplayer could use some work because there is only one gameplay mode, where you must collect the most diamonds before time expires. Luckily they added some new maps and achievements in downloadable content so you will have new maps to explore in multiplayer. It is nice to have leaderboards so you can see how you stack up against your friends and the world in both single player and multiplayer.
All in all, if you have 400 Microsoft Points not being used, spend them on Marble Blast Ultra.