Wow...few games have impressed me more than Elder Scrolls IV: Oblivion.
The sheer size of this game is mind-boggling. How the developers compacted such an enormous game onto a small disc is beyond my abilities of comprehension. Rest assured, if you buy Oblivion (and you should), you will be devoting ALOT of time to it. In all seriousness, you will lose alot of life at the hands of this beast.
My current file is about 30 hours long...and I have done ONE quest that relates to the main story! Only one! The rest of the time is spent...well, exploring the vast world...defeating monsters, interacting with villagers, and completing side-quests (there has to be 1,000 side-quests...they seem endless).
The game is rather complex. You can improve your character in MANY ways such as sneaking skills, alchemy skills, lock-picking ability, etc. Also, during the process of actually forming your character, you'll be amazed by all the different ways you can alter their being. Truly remarkable.
As far as the playability, Oblivion does not falter. The First Person perspective is easily grasped, as are the controls for attacking, defending, and using spells.
Oh, and did I mention that the graphics are arguably the best I have ever seen? Cause they are...
All-in-all, Oblivion is a truly spectacular game that EVERY 360 owner should have. Just make sure you have alot of time to give to it.