I bought this on Xbox 360 Live Arcade when it was going cheap a few weeks ago.
All you do is switch round coloured balls so they match, three or more in a row. Most of the time you play one-on-one CPU battles, where you sometimes use special moves to defeat the opponent, you can also forge items and train mounts in variations on the basic game.
I bought it, I played it, I didn't sleep much, I came back to it even when I didn't want to, and I finished it. And I was glad to finish, because that meant the addiction was over, and I could go back to normal.
This game is mercilessly addictive.
By which I mean that I was playing the game beyond the point where it stops being fun, just as a "default thing to do" - whiling away hours and suddenly wondering where the time went, in the early hours of the morning. Oops!
You play and play and play and then when you're done... you're tired but unsatisfied.
I don't think I'll buy the expansion pack!