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I'm a big fan of Tumi's predecessor to the London Roll, which was named the "Luke" roll top backpack. After the introduction of the London Roll model, it was pretty easy to find the Luke Roll on clearance which may provide a value alternative to buying a new London Roll. Otherwise, consider waiting for one of Tumi's semi-annual sales if you don't want to spend $450.
I did check out the London Roll in-store, and the significant differences I noticed were that the two outer rear pockets were increased to be the same size, and the upper pocket on the rear was modified to be accessible even when the roll portion is fully closed (this is actually a change I do not like, since I prefer to have that pocket more difficult to access).