Well it really depends on your taste. What'd you like on Fable II? Determine this, compare it to Fable III's and decide if it is better or not.
For example, let's assume you liked Fable II's story (seriously? Just using an example). Well you'd LOVE Fable III, it has a better story, with MUCH better voice acting, and character development.
The Weapon System is different as well. No longer will you raise money to go and buy a Steel Flintlock Rifle, which you later sell for a Master Flintlock Rifle. You have the same weapons from the start as the end. Your weapons grow with you. Some do not like this, but PM SAID it was a feature, and he delivered. It's quite the feature, I like it.
The Expression Wheel is gone (yay for me!), and replaced with nothing more than little pop-ups floating around telling you the action and which button to press. No longer will you have to press RB, move the analog, choose an expression, etc; in Fable III you engage in a conversation, and press corresponding buttons to advance through it.
The Leveling is dumbed down, no more collecting experience to spend in a skill tree. You now fight, do quests, and other things for a new currency: Guild Seals. Guild Seals upgrade your weapons, magic, unlocks NEWER features, and other things.
The game has a HELLUVA lot better clothing than 2 does, though not as much. You see, you now have much better artistically fabulous clothing, with a MUCH better Dye System (to some, not so much to others).
The point is (after writing that wall of black nonsense) you should get it if you THINK you will like it. Buy it, try it; if you do not like it get your money back and buy Assassin's Creed Brotherhood.