Would have posted sooner, but I've been busy doing nothing but playing Arkham City.
Without a doubt, if you even remotely enjoyed AA, you'll love AC. Game is simply vast compared to its predecessor.
Finally beat it late last night, and WOW what a story. I was blown away.
And even though I've beaten it, there is still an enormous amount of things to do, including finding all the riddler trophies and challenges, as well as several story elements involving different characters. Not to mention the Combat and Predator challenges.
Seriously, there's so much to do in this game it feel like two games.