Rather than persist moaning about WM10, I decided to just do it and upgrade using the Insider Preview.
1. Performance is generally on a par with WP8.1, slower in some areas, faster in others.
2. Tiles are bigger than I would like, but the "Use more tiles" option makes them too small!
3. Settings app is immeasurably better.
4. Now they've re-added linked inboxes, the mail app is better than the WP8.1 mail app, although admittedly a little slower to load.
5. Navigation is a bit screwy as they've still got multiple navigation methods in use, but it's not as annoying as I thought it would be.
6. I'm yet to decide if the "hold down the Start button to bring the screen down so you can access controls at the top of the phone with one hand" feature is a nasty hack to get around the idiotic decision to follow Android and put all navigation at the top of the phone away from the user's hand, or a clever trick to get around the idiotic decision to follow Android and put all navigation at the top of the phone away from the user's hand.
The upgrade itself went very smoothly. Took about an hour and everything was exactly where I left it when it came back, even down to the Start layout which wasn't preserved when upgrading 8.1 to 10 on the desktop.
Kudos for attention to detail though, as I had the old neutered Office app pinned to my start screen, the upgrade downloaded the new Excel, Word, and Powerpoint apps and put them in a tile group in the same spot where the old Office app was pinned. Not a massive feat of software engineering, but a nice touch.
Overall, the upgrade is much like going from Windows 8.1 to 10 on the desktop, generally a non-event with a few niggles that I'm confident will disappear over time. It's a good start, MS just need to stick to this way of doing apps and not change their minds and reinvent the wheel in a year's time when it doesn't immediately work, because it's not going to.
IF it works at all in turning around the fortunes of Windows apps, I don't think we'll see any significant uptick before the end of next year, as it will take time for companies to evaluate the platform, decide whether to go with it, and then develop the apps themselves.