This has been popping up in rumors and now a few photos have appeared. So, it looks like this may be the next big white lens to appear. It is being hyped as sharper than the EF 300 f/2.8 II, which was one of the sharper EF lenses ever. It my mind, that should make the high MP body crowd happy, as there are only a few reasons to have a lens sharper than the EF 300 II: 1) Extremely high resolution bodies and 2) marketing hype . But fast AF, super sharp, also being hyped as light, and looking at the images, seems very reasonably sized.
I was not looking for this lens, but I could see it as a great second lens in a combo with a 500/600. But, also, adding on a 1.4x or 2x TC, you get 140-420 f/4 or 200-600 f/5.6 lens. So, if it is sharp with TCs, this actually could be a very versatile lens. Use it on a crop body, 160-480 mm equivalent.
Of course, the $10k rumored price tag wakes me up pretty quickly. But, maybe, someday.