I didn't enjoy Oblivion at all. It was all boring and unengaging. Even if Fallout is just oblivion with guns, the difference is that Fallout 3's setting is way more original/encapsulating, the music was beautifully ironic and there was a lot more humour and all of that added up to something that was noticeably more characterized and certainly kept my attention.
If you're gonna do a medieval fantasy RPG setting like oblivion, it has to be superb by now or people will have already been there and bought the t-shirt.