Oblivion is a game I bought after reading many reviews, the first RPG I have bought for the PC, and I wasn't sure at first but after playing Oblivion it earned a 10.
Alough there is some bugs in the game, not very many surprisingly for such a big game, the landscapes are vast.
Another aspect of the game is the ability of being able to obtain new armor, weapons and items of different categories in strength and the ability to put your character up after training and the ability to select a unique character race and character abilities.
The game has so much to do, it will last a few 100 hours to complete it if you ask me, there is alot of missions to do, the main quest or you can do side missions talk to people walking around or join a faction such as the Arena where you fight gladiator style, or the Theives guild where you get missions to steal a certain item, or you can join the Dark Brotherhood where you fufill contracts of hits on people, you kill them and get paid and more respected.
There is many caves and forts throughout Oblivion where you battle bandits, goblins, vampires, bears, wolves or whatnot inside and you can find unique items on bandits or in tressure chests that you need to pick the lock sometimes.
For Oblivion you need a pretty good machine, top of the range to run it as best as possible, a mid-range PC will run it from low-medium even with some lag now and then when outside.