As a gamer who have played and finished first two Dungeon Siege games, I must say DS3 is quite different. While DS1 is a rather dull hack&slash with a little character customization, it has a great party based control system like we used to in Baldur's Gate (of course you have to turned off all this annoying auto attacks). DS2 is much more advanced in terms of story, characters, char customization and quests, but party control was changed with a more Diablo style gameplay.
The first shock I lived with DS3 is the controls. What I expected was a Diablo style gameplay, but it's more like Baldur's Gate: Dark Alliance. Actually much more thar BG: DA. DS3 is the most actiony game in the series. After cursing the controls a bit, I saw that it fits perfectly to the gameplay. With all the dodging, attacking, spinning I saw that I use every button in the keyboard mouse layout. Left mb attack, middle mb camera, right mb running, Q changing the combat stance, space for block/dodge. It all fits well, and there's no better way to put these buttons other than that I think. But, the one thing that sometimes drives me crazy is the camera. Camera zoom in and out to its mind and blocking your view to see the enemies. I think most of the people who have problem about the gameplay stems actually from this weird camera system.
The other thing that I dislike about the game is the corridor feeling. One of the most important feature of the first two games is to traverse different terrains without any loading time and Obsidian did that with DS3. But it's not only that, when you are in a forest in DS1/2 you fell yourself in a vast forest. Not in a corridor whose walls made of trees.
Graphics not impressed me. I hate this bloom thing. And they use that much. Like Two Worlds 2 and Fable 3 they overuse bloom in order to conceal the low resolution of the textures. While it might not be a problem in a console, I want to see sharp graphics in my PC. You can turn off that bloom in TW2 and F3, but I can't find an option in DS3. It's so bad that my character's face is shining like a lightbulb. I hope they will release a high res texture pack in the future with the option to turn off bloom. Another annoying thing for me is DS3 isn't support 4:3 in the game menu. You have to tweak the ini file to do that. But the position of the HUD doesn't change, but it's much better than seing black bars in the screen.
What Obsidian did right with the game is the characters and the story. While the story seems cliche, the presentation is perfect and the characters are believable. It's the best thing that Obsidian/Black Isle does since the time of Fallout 1.
I don't know the length of the game but I heard some comments in forums that it can be finished in 12 hours. If that's the case it's so short. I don't know. I'm still in the beginning of the game.
Overall a decent action RPG, which is more "action" RPG than Diablo style games. Try it and decide yourself. But give yourself some time to get used to controls.