Before I bought the game 2 days ago, I had only played it on the 360. I stopped playing because the lag and spiking made me resent the game rather than enjoy it. I could say the same about Oblivion.
My computer is very decent, I'd say above average. Fallout 3 looks MARVELOUS! Well, at least compared to what I experienced before. Loading is smooth, graphics don't slow down, everything is just more detailed... I'd say if you have a computer that's up for it, PC is the way to go.
On the other hand, I can't offer a good opinion if you've already played the game thoroughly. Also, somebody here has stated that the PC version is prone to crashing? I haven't experienced it yet though, and I've been playing it all day yesterday too...
Half price is hard to argue with
Edit: I've taken a screenshot... Running 1680x1050, High Quality, Soooooo smooth. Apparently, my screenshot is too big to be attached via forum. Screenshot
here