Australian retailers broke the street date today so I had a chance to buy it. Loving the game so far. I found Batman: Arkham Asylum (AA) to be a little frustrating and repetitive, though I did love the game's presentation and feel overall. That said, Arkham City (AC) goes far beyond what AA did, IMHO. As you already know, in this iteration, Batman's playground in a sprawling, decaying urban jungle *much* larger (or that's how it's feeling so far, anyway) than its predecessor.
Visually, it's more or less the same. But then again, AA was smaller in scale, and to retain much of AA's looks (and even bests them at time) in a bigger setting is no small feat. Combat is also similar, but the "quick attacks" you get to use with your weaponry when you f!ght string combos better.
Overall, it's better than I expected. I didn't LOVE Batman: AA like everyone else did, but I did enjoy it as an action/adventure. Coming into AC, I knew I'd like the game, but I didn't expect to like it this much. Gliding around the city, grappling onto buildings as you glide, is a welcomed mode of transportation. I guess you actually feel like Batman a little more in AC; you're placed in a city setting and you're out there, finding dumb scum to drop on before you glide along to your next objective.
If y'all have any questions, just post it here. I'll be glad to answer until you get the game in a day