I just finished playing Fallout 3 (non-GOTY) and thought it was quite excellent. I've never played Oblivion or Mass Effect, so I can't comment on how it stacks up against those games. I did hit the level cap of 20 before finishing the game, but not too much before. Honestly, I stretched out the game by deliberately not finishing the main campaign and doing side quests, just 'cuz it was so darn fun.
Initially, I was very excited about getting the DLC packs. "Broken Steel" raises the level cap from 20 to 30. Then I read about how buggy the PS3 and PC versions are. Some people have had a lot of crashes or slow loading. I don't know about 360 version. Bethesda apparently still hasn't released a patch yet.
Anyway, I'm looking for a cheap GOTY edition for PS3. I'm hoping that I don't experience the bugs like some other people have.
I've bought a few games on eBay, and have not had any problems. YMMV
EDIT: I forgot to mention that even the original game (non-GOTY) freezes up on the PS3 sometimes. For me, it's annoying but not gamebreaking.