I've played the 360 version of Dungeon Siege III for about 6 or so hours so far. And, well...I like it. It reminds me very much of the Champions of Norrath and Baldur's Gate games that my wife and I had lots of fun playing.
As for the first two Dungeon Siege games, I did not play them. I did, however, watch my wife play them. She beat both of them, but doesn't remember much of the plot. I know this because not only does she forget things, but I tried asking about some references in the third game that I believe to be from the first and/or second. I was told in no uncertain terms that she didn't remember the plot because she just wanted to go around and kill things. And drag a mule around to carry the extra stuff.
That said, the third Dungeon Siege seems somewhat similar to the first two. View is the same, and it is a dungeon crawler/loot game. The first two were very much like Diablo, in essence. The third, being developed for consoles (or at least on them), has a much more action-oriented style. So one button for attacks and the other 3 do skills when you buy them at certain levels. The skill button placement is pre-determined, so it does not have the flexibility of Sacred 2, but it works.
What separates this game is that each character has two different weapon styles. For example, the gunner girl has a rifle style for distance and a shotgun/pistol style for close combat. To switch between these, you just push a button. They can be changed very quickly, so it's easy to use a special and switch styles to use another.
There are 4 playable characters, each with a different origin. They're not really fleshed out or well written characters, but its a step up from create-a-character's story. And so far, the story and quests are well written and enjoyable.
Now for the co-op. This was a part that a lot of people really hated when talking about this game. While it would've been nice to be able to play in another's game with your character, it's not so bad to just be their level and draw from their equipment. This is very much old-school co-op and for the couch, it's not too bad. Going online, I can definitely see how someone would want to keep story progression and equipment, so it kinda sucks for that. Playing occasionally with a buddy or significant other, the co-op works just fine.
So all in all, I am liking the game. It was easily worth the $10 I spent on it. If you like action rpgs, I would recommend picking it up for a low price.