After 4 months of use 10 windows mobile, I can say that it is ok but Windows Phone is far better for my needs after long use both systems.
I'm going back to Windows Phone with my Lumia 640 XL and here's why:
* With W10M my camera did not work ok when it is supposed to take images of the highest resolution supported. Camera was resuming with the camera app I could not use camera app for video when it is used to record in the best quality at 30fps, and also lower quality . I had resuming and app Camera could not be used, I jhad to do reset on build 586.
* Windows 10 is slower, WP 8.1 is still flying pretty fast. Tiles animation is better for me.
* Some applications run slower on w10m such as 6tag, viber - notification and display incoming text messages on Viber. The handset and SMS to open more quickly.
* Cortana is much more stable on WP8.1 and faster. On w10m is crashing.
* The action of opening apps is too slow on W10M. Some apps does not start on click on tile, sensitivity of the screen is not good on w10m, on wp 8.1 sensitivity is better. I don't see a reason for that.
* Return or windows button are reacting faster on wp8.1. On w10m is resuming everything and all the time.
* Applications like WhatsApp and Viber are the same on this systems, only the FB is different, but here in wp 8.1 runs faster and better. Viber is not slow.
* Notably longer lasting batteries with wp8.1
* MUSIC is a far better player from Groove. Music allows you to make a playlist via computer using winamp a groove just do not see this list created from computer. It is difficult job to someone like me who likes to combine songs and must hand the phone to a song by song added to the list.
* Touch on screen better respond on wp8.1, double-click the screen turned on phone screen while on W10M not turns on the screen.
* wp8.1 is organizing photos better and according to date and not in hundred folders.
From w10m I miss the wordflow on Serbian and useful Action Center.
I also like what W10M has a background behind the tiles that can be transparent.
I'll see, I will test wp8.1 in next days. But unfortunately in my experience w10m is not satisfying. w10m is not fast enough and not so stable.
While on wp8.1 never knock anything and everything is going very fluid and fast.