Let's start by saying that I HATE Diablo 2, but I loved Dungeon Siege 2, so you can tell that the 2 games really are very different. DS2 is so much better than DS1, so if you liked DS1 it's pretty sure that you gonna also like DS2. Yes, there is still lots of hack and slash, but less than DS1 and far less than Diablo 2. The story is much, much richer than DS1, the music is wonderful, the graphics are just as good as one can expect from a modern RPG (which means lots of details, effects, shades and whatever the game developers can think of so that to take our graphic cards to the limits; I'm not always fond of this stuff so I reduced some of the graphic details, although my system could handle them pretty well).
But the gameplay is the best part of the game. Lots of quests organized in your journal (you can even know how many more quests, optional and not, remain until you finish the game), easter eggs, books and history to read and other very cool stuff that make the game far more different than the average action rpg. It's just the kind of RPG I like to play, not too action nor too story-oriented. DS2 has both at equal amounts.
Concerning the expansion pack that comes together with the box, is not that great as the original game, but definitely not bad. So, if I would rate the original game with 5 stars, and the expansion with 4 stars and keeping in mind that I have found the whole package is currently being sold for 10-15 euros, yes, it's an absolute must buy and the purchase deserves the 5 stars.