So I ordered a couple of 64gb black OPOs on Tuesday (Chinese flash sale) and have been using one since yesterday morning. I'd already handled a couple that other people had but here are my impressions from a couple of days' use:
Design: bezels are a bit bigger than I expected but not too fat. The metal frame exacerbates the non-screen space a bit. Top and bottom areas are pretty porky but at least they curve to their bulge! It's a big device - that much we all know.
Ergonomically, the curvature of the back is good. I have fairly large hands and am used to gripping my 5s and M7. However, it's pleasantly surprising to say the OPO is quite nice to handle. Aside from the back curvature, the other major factor is the sandstone texture. When I tried the other two white OPOs I found them a bit slippy and slidey. I'd go as far as saying the sandstone texture is the best finish to a device I've used - no fingerprints or dirt and excellent stability without having to grasp it.
And to the yellow: both devices I ordered have a tiny amount of slight yellowing, about 3mm from the bottom only noticeable if I tilt the phone forward at a weird angle. Apparently, CM runs warm so maybe if I was in this ROM I would never have noticed. As it is, I would say it exists but I had to go looking for it. Either I got lucky (or unlucky?) or there has been some change to the manufacturing process. It's certainly much less than the prior two OPOs I saw and not nearly as yellowy as my 5s is (from top to bottom).
Software: ColorOS is quirky and not to everybody's taste but I like it. Sometimes I feel CM is a bit too boring. I know, I'm in the minority here! The biggest reason I haven't yet tried CM (I will) is the camera: by all accounts the CM camera is bad and the test shots I've done on ColorOS are very nice. I guess CM is the same but ColorOS on this is on par with iOS smooth wise.
Battery: About 6.5 hours screen on time with plenty of restarts, benchmarking and installing. I need to let this cycle a few weeks first but it's promising as I'm a relatively mild user (my 5s gets me 1.5 days)
Overall, it's good but not perfect. It cost me circa 257 USD thanks to an impatient colleague so I can't complain too much. A nice play-thing and I'll see how it performs over the next month before the big glut or new phones.
TLDR: Overall it's not perfect but is unquestionably cheap for what you get by comparison. A touch large and there's room for improvement but the black sandstone version reduces handling issues. If I was buying outside of China, the support/returning side of things would concern me more as well as not having flash sales to get one quickly and reliably. Oh, and don't bother with the 16gb model. As a harsh critic of OPO it is a good product, just less so the company.