I have been using Windows 10 on my Lumia smartphones for the last 6 months. I'm very much impressed with the hard work done by Microsoft. First Windows phone I used was with Windows 8 which was a bit different and difficult compared to Windows 8.1. However coming from Blackberry Curve 9300, it was a great welcome OS. The Windows 8.1 stole the show with very fluid, stylish and simple design.
Windows 10 mobile started off with lot of bugs and hits and misses, but that is how any development work happens. The difference is that Microsoft chose to provide these builds for public use if they join the 'Windows Insider' program.
I became an insider and have been enjoying Windows 10 since then. At first, after a few days of experiencing the builds I switched back to Windows 8.1 considering that I had only one Lumia then and I was using it for the office work. As the Windows Insider program states that those risks shall be 'understood' and the primary phone is not recommended to be used with such preview builds, it was correct that I switched back to Windows 8.1 using the 'Windows Device Recovery Tool' software by Microsoft.
This particular software got its name changed many times. At first it was being called Nokia Phone Recovery tool, Then it became Windows Phone Recovery tool and the last change to its name was Windows Device Recovery Tool. It is very simple to use. You just need a USB cable to connect your smartphone with your computer.
But as soon as I purchased my personal Lumia 535 phone and Lumia 435 that came later, I installed Windows 10 preview builds. But to my surprise the builds were already better. Even after that once or twice I changed back to Windows 8.1 when I encountered few bugs, some lags in performance of the phones. But I couldn't resist coming back to using Windows 10 preview builds.
I think that speaks of my experience why I liked Windows 10. At the background many things might have changed technically. But to an average user the changes to the interface and simplicity are more relevant. I'm no average user, but still the new features added to Windows 10 like transparent backgrounds, many notifications with expanded views at the top of the phone screen, clearly organized settings, great improvements to 'Mail and Calendar' app, Cortana, and few other features were so interesting and so exciting to use that I couldn't go back to Windows 8.1.
And considering that Windows 10 got released officially for the PCs in July 2015 and later on, the compatibility with the Smartphone as well as the PC got greatly improved, and the Apps were the same across my devices, there was no question of going back to Windows 8.1.
Then when I got Lumia 640 XL two months ago, the first thing I did was start the phone, once the initial set up was done, I was quick to cancel all other app downloads, straight away installed 'Windows Insider' app and installed Windows 10 technical preview build at that time.
To sum up, Windows 10 Mobile is going to be a success just looking at how good the operating system is, for both PC, Smartphone, tablets and any other Windows device. App-gap still haunts Microsoft mobility business but those issues don't take away the credit from Windows 10's versatility.
Now I have all my family Lumias installed with Windows 10 builds despite knowing the fact we compromise on some security and we are 'vulnerable' to risks as claimed by some 'seemingly anti-Microsoft' blogs. But to me, such risks are yet to be encountered. For daily use, the current Windows 10 builds for smartphones are already better.
We can comfortably wait for the 'best' build but for the moment, I'm quite impressed with the latest build. Windows 10 Mobile is almost ready but not official. So try latest build 10586.63