About a week ago, I downloaded the Xbox 360 demo and played through all of it. The graphics were so good that the frame rate would go down sometimes. The new gameplay feature with the combo attacks is neat. It goes in slow-mo while fighting someone and you have to press each button shown on the screen to perform the combos. Seems like it's going to be a great game.
I downloaded the demo on my 360 and never noticed any drop in frame rate. It played perfectly smooth all the way through, and I've played it a few more times since then.
I just rented it for the PS3 and the frame rate issues are the same that Lance mentioned but overall the game is really fun but its kinda short. I'm almost finished the game on Normal and it took 2 - 4 days I think