If it's any consolation, I tried Oblivion and hated it, but I really enjoyed and beat Fallout 3.
I thought this as well. Oblivion pissed me off because it never seemed clear why I was doing anything. Some guy releases me from a jail and now I'm outside. Ok so who am I, what am I doing? What's up with this place? I didn't know enough about the situation to care what was going on.
Fallout 3 is a bit of the opposite. It has characters in it. It's clear what you're doing. You meet people along the way that are memorable and say their own unique things.
Overall, I think you'll like Fallout 3 if you like to dick around and not actually do the story. Grand Theft Auto Vice City was one of those games where you can spend hours just roaming around, crash cars into things, shooting people, looking for secrets, looking for guns, etc. Fallout 3 is a lot like that idea. The story is a very very small part of Fallout. There are lots of places to explore that have nothing to do with the story, but they seem interesting because they give some perspective of what this post-nuclear world is like. There are caves with bears in them, caves with deathclaws in them, caves with raiders in them, side quests involving killing a bunch of twilight style gay vampires, other vaults with interesting back story to them, etc. I also liked roaming around and shooting rockets at every car or train I saw.
TLDR: it's an adventure game. If you like to go on a treasure hunt, you'll like Fallout 3. The RPG and FPS elements by themselves are horrible, and the game is unstable. I love the game, but I can't even play it right now because it keeps crashing and I don't understand why. It worked fine and could play for hours at a time, then suddenly it doesn't work and crashes every 2 minutes. It's completely random like that.