I have to say that Fable is a worthy game for purchase, if you go into the game without any preconcieved expectations, and enjoy it for what it is...you will really like it. Its a great action RPG with many unique properties, but its not the best RPG on the Xbox by a long shot, and if you don't take the time to smell the roses in Fable you can have a very short gaming experience. If you take the time to really immerse yourself into Fables game world, and try to experience all the side quests, different character evolutions, ect it can last you a good 40+ hours, however if you just blaze through the main quest and never experience the different types of characters you can eventually become, your looking at a 10-13 hour game.
If you want the best RPG on the Xbox stick with Star Wars KOTOR, or even Morrowind GOTY edition.