I saw one of these a few months ago, it was the non-IS II-version (I was a bit excited because I thought I'd spotted a pre-release of the IS II).
The one thing that would put me off it is the weight:
400/2.8 USM: 6.1kg
400/2.8 USM II: 5.9kg
400/2.8 IS USM: 5.3kg
400/2.8 IS USM II: 3.8kg.
So don't expect it to be hand-holdable, make sure you get a nice Gimbal-head or Physiotherapist. The non-IS versions also use the 48mm drop-in filters, the new ones take 52mm. If you like using CPLs, take that into account because you may or may not be able to find any.
As for the IQ, I have no idea, I've never seen a review (and if there is one around, I'd love to read it). The price sounds very nice, as long as the glass is clean and non-fungal and the aperture works. Is there any way you can test it out before you buy it?
I also wouldn't mind the lack of IS, I'd also be using it wide-open if I could get one.
hmmm, actually, that price looks very nice, if I had the cash I might even consider out-bidding you on it...