After more playing around on the 5D III I am even more impressed by this lens. Its just so sharp wide open, with bags of contrast and is so incredibly responsive on the new camera. Testing shows it is a bit weaker at 70mm than shorter focal lengths at f2.8, but I think its not an issue in the real world. This is the SLR zoom I have been waiting for. I spent some time familiarising myself with the lens and the new 5D III at the seaside with my kids. Colour, contrast and sharpness are all absolutely stunning. Focus tracking and speed was incredible. Its quite a new experience to be able to shoot at f2.8 on a zoom and not remotely worry about optical performance, only depth of field. I got frames that really blew me away. I kept checking to see if they were soft, but almost all the time there was the focus: smack on, with loads of bite and contrast.
I don't think the pancake is up my street really, but it does sound a good lens. For that sort of FL, I'd put a 35mm on a 35mm RF. To me, SLRs are for zooms and lenses longer than 50mm.
Next stop: TriX.