My experiences with W10M preview:
W10M preview works very well on 640, battery life is very good. Mail, SMS, phone calls, apps - they work.
Responding to SMS in notification is very good feature.
UI - one can like it, but I think that generally it is just too small. What works in PC screen does not always work in phone screen. Phone is mobile touch device after all. Making number pad (sim pin, phone number) area uniform (without borders around "keys") is bad. Hitting buttons and ellipses, closing alarm etc are not as easy as in WP8.1.
One can increase scaling but effect is not enough and other parts and apps become too large. Lumia 950/XL are not so much bigger than 640. Makes me wonder if they test usage with actual users (formal UI testing in addition to insiders) and actual devices at all. Old-timers (age > 35) beware

Scrolling is jerky with occasional jumps to warp speed and re-acceleration in the middle of deceleration. WP8.1 is also sometimes jerky but never as much as in W10M.
3rd column in start screen can be removed. Default font is thin and not too easy to read from tiles (even with 2 columns + increased scaling). Alarms live tile shows next/active alarms.
Still no Weather on lock screen. Some other app might work there. Hard reset -> Here Transit is gone for good. Maps has similar function but it is not as usable. I'm using Sygic and Here Drive, haven't tested Maps navigation yet.
Camera is a grief... Live images is very unpredictable, selecting best lighting for pictures taken with "super" (the magic wand) worked for flash intensity and 1 picture (out of dozen or so) for HDR. "Super" images sometimes come out as 1280x720px. I'm not sure if Lumia Camera living images are live in W10 photos or only in phone...
One can live with W10M in daily usage, but because the things above bugged me too much, I chose to revert back to WP8.1

I previously ran 820 with preview for more than a month. 640 jumped to preview on second or third day for a couple of weeks.
When 640 is officially updated to W10M, new fixes and new firmware may make it better. Problem is that at that point there is no way back to WP8.1...