If you want to have some fun slicing and throwing people to abysses, this is a great game for you.
-source engine
-fun gameplay and orcish dialogues
-Xana and option of being evil :)
-nice multiplayer
-using bow with flame is always fun
cons:
-too easy after you are used to the game
-easy and unsatisfying boss fights
-does not use source engine's power for outdoors. It's mostly Arx Fatalis-like and it gets boring to fight spiders inside caves million times.
-boring books. do not ever read any of them
Dark Messiah is a good game if you think it as a fps game, but if you're a RPG guy, you just have to stick with oblivion. Dark Messiah's RPG part is not complicated but it is enough for many people. You spend a point to an ability and it can be up to 3rd level. You can use weapons and armors according to that. Poison deals too much damage and you have to drink antidotes too much but it's never sufficient, so I strongly suggest you do something to regenerate health points.
The scenario is not %100 linear and you sometimes have options like killing the big monster or run away. If you want to follow the scenario by reading books and finding secrets, don't bother. Just focus on killing enemies the most creative style and have fun.