Thanks for the review Dustin, I've owned both the 28=350mm L and the 28-300mm L IS. They are wonderful lenses, but heavy and the aperture only makes them suitable for at least moderately good light.
I do wonder about the even smaller aperture of this lens.
I think its great to see so many affordable new FF lenses that are ever improving in quality. This will allow more and more photographers to move from crop cameras to full frame without fear of having to spend a small fortune on new lenses. It also puts pressure on the OEM's to reduce prices and improve quality. That will happen if there continue to be more and more competitive lenses.
Nikon FF users should be very happy, since Nikon FF lenses are priced very high. They are probably feeling this more than Canon is.
I'm not crazy about f/6.3, either, but it is also 1/3rd of a stop of light. It is rare that the 1/3rd stop makes much of a difference, and the high ISO performance of the 6D that I use it with means that I can get away with cranking ISO and still get very nice images.
I suspect that the 28-300L probably has higher resolution than this lens...but I doubt the difference is significant. This lens is surprisingly competent (I wasn't nearly as impressed with the new 16-300 VC for crop).