I think anyone who hasn't played it probably has a reason not to ever play it. Personally I have and I really like it, but it's been out for a while and gotten cheap ($5 for GOTY on Steam recently) so there's no reason not to have bought it already if you want it.
I have beaten Fallout 3 a number of times, but it really isn't a game for everyone. I have the game for both PS3 and 360, I didn't know what kind of glitches were in the ps3 version, or I would have steered clear of it. I wouldn't really say it is my favorite game of all time, but it is really far up there.