OK, so a quick update; the lens arrived 2 weeks ago and we had a play last weekend and again this weekend. On both weekends, the light was not ideal, so we were shooting at up to ISO4000.
The lens itself seems very well built, not too heavy (although my other half, who is using the lens on her Canon 6D may disagree!). So far, focus lock has been quick and accurate, bokeh is handled very well and the OS/IS/VR (whatever you want to call it) is quite effective, probably 1 or 2 stops effective (up to 4 stops claimed), but that will be dependent on how steady your hands are to start with. Anyway, so far it has only been used on either a tripod, mono-pod or beanbag for support, not tried to actually handhold it yet. Results have been OK so far. I say "OK" because i don't think we have tried to use it in the best light yet (first time was in a dark forest shooting bird-life and squirrels, and second time there at the weekend was on an overcast early dawn at a local bird sanctuary. Attached is one picture from the bird sanctuary this weekend. Very happy with the sharpness given the light conditions.