I agree also. Francisco clearly has a vested interest in this opinion, and like you say he's absolutely right about implementation making the difference.
I have given a Cappuccino app to people to use, and they didn't even realise there was a difference from the native controls (after the widgets rendered - which DID throw them).
Even though I fundamentally agree that re-implementing UIs from scratch is not optimal, Cappuccino had a valid reason for doing so, and its implementation doesn't harm usability.
With the current 'jake' branch and its' imminent merging into master, the framework resources are sprited meaning they all load in one http request (rather than many). This eliminates the fragmented loading of the UI, which, I agree, can throw people at first!
I have given a Cappuccino app to people to use, and they didn't even realise there was a difference from the native controls (after the widgets rendered - which DID throw them).
Even though I fundamentally agree that re-implementing UIs from scratch is not optimal, Cappuccino had a valid reason for doing so, and its implementation doesn't harm usability.