I have the PS3 version, and honestly I haven't incurred in any framerate issues so far, its been running smoothly, and the graphics difference between the 360 and the PS3 version? Well thats some B/S whoever posted the screenshots on the difference between the two systems in graphics then they must be on crack, no difference!
Now as far as the game goes, I've been little deep into the whole story, its a game like Oblivion, you have to get deep into the story to get into some good action, other than that, the beginning is very boring, but I guess if you like games like Oblivion, its anything but disappointment. For me I am already tired of it, I went back playing Motorstorm

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I thought initially Assasins Creed was the game we all thought it was, but the game itself going in and out of the animus, well thats just time consuming, a lot of parts get repetitive. If I were you I would rent the game first for a week or so and see if you like it or not, If you do go ahead and pick it up.
Hopefully this helped.