First off: This game is NOT a bad game!
It actually is a pretty good one. The problem it has is its name. This is no Dungeon Siege!
Obsidian/Square Enix would've been better off by giving this child a different name, or make it a Spin-off (like it happend with Baldurs Gate back then. PS2 version was called Dark Alliance). This would've been alot better and the die hard fans of Dungeon Siege 2 wouldn't have been dissappointed that much. Especially when considering that they made this game with consoles in mind. The Port is bad. Crappy and slow UI, plays best with a controller and so on. Performance is good though.
Anyways, in terms of Action-RPGs, Dungeon Siege 3 is not bad. It has its flaws, yes, but overall it's good enough to be picked up by fans of the genre. Especially at a discounted price.
It features good graphics, a decent story with multiple choice stuff, okay-ish combat with dodge mechanics and a nice lootsystem which keeps the good loot coming until the end. The companion system they used in the game felt kind of good too. They weren't annoying and could stand their ground on their own. They even pick up gold drops for you.
The biggest problem this game has, is its lack of bigger maps. It's mostly a really long tunnel... and I mean really long. It's on par with FF13. There is not much exploration going on and this is especially bad when you reach Stonebridge (the games Mainhub/Town). It doesn't feel like a Town at all.
Where the game kind of got it right was with the Treasure of the Sun DLC. Here it actually feels like you are running around on a big map with ways to explore it. Still a longshot away from being called an open world, but it felt satisfying to run around in the Desert. The DLC introduced a few new/nice Puzzle-/Dungeonmechanics as well.
So getting the DLC with the game should be a no-brainer.
But I don't recommend picking it up at the current price. 20 bucks for the Game+DLC is too much. 10 bucks at most seems about right.
All in all I'd rate it with 7/10. So definetly recommended to fans of the genre.