[A posted review of my new toy]
This is my third Windows Phone (Lumia 800 to Lumia 925 to Lumia 950), and each iteration has - in my experience - been better than the last.
I delayed upgrading from my Lumia 925 because there appeared to be some downsides with Windows 10 Mobile (namely that it seemed to be a work-in-progress), but in the end I needn't have worried. While it's not perfect, it's a solid step upwards with only a few deficiencies relative to Windows Phone 8.1. The only noticeable glitches I've seen so far are occasional - and temporary - freezes, where it feels like the phone may be waking up (a legacy of W10M's origin with W10 on desktop PCs?), and the app-killing interface being slow - though this has the feel of being designed-in (= a future fix?) rather than an actual glitch. In any case, as W10M shares the kernal of W10, I would expect that Microsoft will be giving W10M much better and more consistent support than it did WP8.1 (which was still a pretty great OS).
As for the phone itself, the only downside is that its body - wholly plastic rather than part metal - is a little less premium, though I was well aware of this aspect before purchasing. The rest of the hardware is great, with a great screen (including Glance and fantastically dark blacks), an excellent camera (though this has been the case with Lumias since the start), a replacable battery (no equivalent in Lumia 925), an expansion slot (no equivalent in Lumia 925) and a much zippier GPS sensor (judging from the performance of the sat-nav app). As the camera has also been slightly repositioned, it's also nice to finally be able to use some little lenses that I've got with it (and, finally, a selfie stick).
One brand new feature worth a mention is so-called Continuum. This lets you plug the phone into a small dock and then connect it to a monitor or TV (e.g. via HDMI), and then use it as if it was a PC. While not all of the apps work in this framework, a lot do, and it has potential if you don't want to lump around a laptop or tablet computer. One of the most convincing things I've done with this is access and interact my desktop computer via the TeamViewer app - this works just brilliantly. If you're a business user, it's worth seeking out a demonstration of this feature, as it's really quite unique. (I should quickly add that the dock isn't free - unless, that is, you're able to take advantage of the same Microsoft deal that I was!)
While the app gap remains (if this bothers you; it probably doesn't by now if you're with Windows), it feels slightly narrower with W10M, and I've noticed upgrades with certain apps that never made it to my Lumia 925 (e.g. Instagram, sat-nav). Most of the apps that I used on WP8.1 on my Lumia 925 seem to have transitioned well to W10M, and I've found a few nice new ones to fill some minor gaps. The only omission I've felt so far is that the post-Nokia sat-nav app - while great, and much faster than before - doesn't appear to have the traffic function of its Nokia precursor.
Anyway, overall, I'd really rate this phone. It's not perfect, but no phone I've used is. And the unique interface that attracted me to Windows in the first place still stands out (and is pretty much better than ever). But be aware that the body isn't quite so premium as earlier models, and that the app gap hasn't much changed. But if you can overlook these relatively minor downsides, it's a definite step up from previous models.