early reviews called it a cross between Diablo 2 and Half-Life 2, and that wasn't far off the mark.
it is from the creators of Diablo (since done with Blizzard) and the back end of the game is very much in the same vein. the bulk of the game is a shooter (first/third).
online is a ton of fun, and after two very major patches i feel it's worth buying.
but don't buy the limited edition, it's not worth the extra $10.