Upload & Sell: On
I'll get one or two when I get another camera in a year or two. For now, buying a 600 for my 5D2 gives me few advantages for a lot of extra cash.
I use mainly on-camera flash for my 1Ds2, 60D, and 5D2 -- very rarely remote flash. So the 600 is not an improvement at this point. When upgrading to a new body (next gen?), I'll upgrade flash. For now, I use 550EX, 580EX and 580EX II.