On the list at my shop so could be first on my block.
To some of the reasons not to posted above, the availability of panning AF as already been clarified by the OP. To the point about the 100-500 + 1.4xTC, if weight is your only concern and you're happy with the comparison to an overly long and unweildy 420-700mm lens...
If I see any lens specific issue for me so far (and I've been away at a family wedding so I haven't had a chance to really listen to a hands on review) it seems to be the amount of turn in the zoom ring to get from 200 to 800mm. Sure, that's a huge focal length difference, so I understand the need to do multiple cranks, but it seems like it requires almost two full turns to get from min to max. If I think about the first turn as 200-500/600mm and the second turn as the rest of the way, as if it's adding a zoom onto my zoom, it's easier to accept, but not what I'd consider ideal. Would I be happy to pay double for a fix? Probably not. LOL
I can't see this replacing my 100-500mm. But in certain situations I can see using it for wildlife in the field, and the idea of a 320-1280mm with no TC on an R7 in certain situations is beyond appealing. The one thing with that, which was mentioned in one of the two reviews I've partially watched, is that you cannot overlook temperature distortion when shooting outdoors over distances where 1280mm is required to get you to your subject.