Thank you, Ross and Colin!
There is very marginal loss in sharpness when using the lens wide open with the extender. Nothing that would be picked up in web resolution, or even a large print, I'd imagine. It is there, but, it is very, very slight. Thankfully, this incredible lens is tack sharp to begin with -- its MTF charts are off the hook. This coupled with the new 24-70mm f/2.8L II USM (its MTF charts are equally as badass) make a formidable combination.
I recently shot a wedding with it and was processing the images. Even when viewing thumbnail images taken with that lens in DPP, you can just pick the shots that were taken with it. It has a look and feel to it which is hard to put into words (possibly the excellent microcontrast). It just works.
H