If you enjoy RPGs because of a coherent, evolving, deep story, with interesting characters, locations and quest then just go ahead and pass over Fable. Forget all the hype about revolutionary mechanics, nothing in that game is, nor will be, even with added material. If you think KoTOR is a weak RPG than Fable will welcome you into it's arms and whisper into your ear that it is the anti-christ come to sodomize your love for the RPG Genre, also Peter Molyneux might stop by for a three-way.
But if all you want is a fine Action Role Playing Game well then I guess you better go ahead and buy.... The Legend of Zelda. Any one of the long standing series is head and heels over Fable, and yes I'm including zelda 2: Links awakening.
Seriously it's that bad and if you read the xbox reviews you will find an interesting pattern occur, none will comment on the broken aspects of the game, doors that lead to nothing and some that don't open. The fact that the game is 20 hours MAX, about 5 to 8 if you stay on top of what you're doing, you're not very well rewarded, there's no real challenge some of the spells don't work properly and some animations is stiff, yet it gets good rating all for the same reason: It's fun. Bad news, it's not that fun and last time I checked fun games don't deserve near perfect scores when they're a haggard mess of what they're supposed to be.