Unless there are significant improvements I will be hard pressed to upgrade to the MKII. The MKI is already a good performing lens.
Version I also has an issue of calibration. I bought one 3+ years ago, and sharpness was much poorer than my 70-300mm DO! Sent it back to Canon for calibration with marked improvement.
Since weight is an issue, I sold the original 70-200 IS f/2.8 (very sharp) and compared the 2.8 II (as tested by Lloyd) to the f/4, which is fine. BTW, focus is very critical. I eventually shot tethered with a small laptop and manual focus at 200%, and could very quickly manually focus and run through of series of f/ stops with the best possible focus.
I think the take home message that any lens needs to be checked out with the expectation that a good number will either need to be returned or sent for calibration under warranty. Really too bad that we have to do all that work to get a good lens! But once it's checked out can forget about it and enjoy shooting.
Paul