For South Island NZ residents, Yoobee in Riccarton Mall has one on display (base model). It's exactly as I assumed it'd be.
I have to say it was a bit clunky here and there. Scrolling through the Apple website on Safari was very smooth indeed. But some window opening was just slightly slow on the redraw. Not terrible at all, but you could tell it was working hard to display all those pixels. Otherwise the machine was responsive and quick, although not exceptionally so. This is the base model, though.
Otherwise the screen was as good as expected (if you're used to a rMBP, it's like that). And disappointingly small. It would have been so much better to have gone up to 30-32 inches. It's going to be hard to go down to 27", but it is already hard going back to a low-resolution screen after using it for just twenty minutes or so.
No standard option for going native resolution -- the "best for display" is the standard HiDPI middle option. Presumably gfxcardstatus or similar will allow us to run at native resolution. I think it's quite doable.
They had Parallels running a Windows 8.1 virtual machine. That was quite snappy with just two cores and 2Gb RAM, and of course it was running within its window at native resolution. Very nice indeed, and very small but quite usable. I can imagine running at native resolution a fair bit of the time.