To close the loop...
...placed order on Wednesday late morning, had device in hand by Friday afternoon which I was quite impressed with.
Overall impression is that it is a quality piece of kit and that it is a (small) step up from the DK2. Packaging and hardware feels like it's designed for prime time.
I don't find it as comfortable as the DK2 - to get it positioned properly I need the top of the face surround to be resting on my brow. Perhaps I'm slightly neanderthal in my bone structure but this is a little uncomfortable but perfectly bearable for the few sessions that I have been able to play (only 30 or so minutes at a time so far).
I don't really notice the reduced FoV unless I'm looking for it, and certainly when busy it's not remotely perceptible to me.
God rays are there but don't bother me overly - in general I'm busy doing stuff and they don't even factor in my perception. I can see that they would bother some and, of course, I'd prefer it if they weren't there at all but in general it's fine.
No problem with head tracking - that feels crisper and more reliable than the DK2 - perhaps due to the increased frame rate.
Visually, it's a small step up. The SDE is still noticeable but again, only if I'm looking for it. Biggest improvement, but still with some way to go, is the clarity of the text and HUD display items. Sadly the red pixels are still a problem. I switched back to the classic HUD colours and whilst it's a big improvement in general clarity over the DK2 it still feels like there's further to go.
Finally, I had no problem getting the device connected and recognised by the computer. The runtime did some firmware updates when everything was first connected and I ran through the setup without a hitch. The debug tool was showing a solid 90 FPS everywhere which was as expected.
I then fired up Elite and had it crash on me 2 or 3 times, then when I did manage to stabilise things no matter what pixel density and graphics options / in-game SS settings I chose (even with 0.5pd and 0.65 SS) I couldn't get above 77.7 FPS in Riva Tune or in the debug performance monitor. Figuring that a GTX 1080 ought to be able to outperform the settings on my GTX 970 setup, I uninstalled the Oculus runtime, rebooted and reinstalled from scratch.
This worked without a hitch and I'm now getting 90FPS in game. Some tweaking to do with graphics option settings and perhaps a mild overclock but I'm actually reasonably happy.
So the $64,000 (or £530) question: is it worth the upgrade? In my opinion, just about. The performance and quality improvements are not worth it on their own, but knowing that I'm future-proof for the time being and that I managed to get a fair price for my DK2 makes up the difference.
TL;DR - moderately impressed with the upgrade, some teething problems which were resolved. God rays and FoV not a huge issue for me. No buyer's remorse. Well, not much.
Cheers,
Mark