Risen is an interesting cross between Oblivion and Baldur's Gate, but is so buggy and clunky that I gave up on it.
Not much need for an overview on this game -- it's an open world, Western RPG with the usual mix of might and magic. I can see why people enjoy it, but for me, I couldn't get past the negatives.
The good:
+ Big, lively game world
+ Lots of questing, loot, monsters
+ Plenty of little hidden caves and caches of stuff to reward explorers
+ Combat rewards real strategy versus random clicking
+ Good voice over work and music
+ Good pacing -- it doesn't get too easy too quickly, you really get the feeling you're earning your skills, items, and experience points.
The not-so-good
- Buggy: the game will delete saves seemingly at random (but it keeps backups you can manually copy back in place -- but why should I have to?)
- Buggy: key quests can get into bad states and you have to do odd things (e.g. pick pockets) to jar the game back into a good state and continue the story
- Lots of trekking back and forth on the map to complete fetch/kill quests
- The game will let you wander to the point that you can completely hose quests
If you love western RPGs, are patient, and can put up with buggy quirks, you may love Risen. Otherwise, better to steer clear.