I recently bought this game and completed it in 1 sit through, the game is quite lengthy and took me 9 hours to complete, once completed you unlock a new difficulty setting which would probably extend the game length to at least 20 hours due to you being hammered for more damage. The AI is actually quite amazing aswell compared to other games i have played. The story is good and i don't have live so didn't buy it for the multiplayer but apparently it is a bit unbalanced . Gameplay wise it is quite challenging on the first play through and i didn't have any problems with the bosses (this might change with my next play through on the unlocked difficulty), You have your Glaive and Weapons and then your skills like being able to turn invisible and using your glaive to fetch different elements such as ice, fire and electric.
Worth the £6 or however much, glad some people recommended this to me