I finally decided to take the plunge to upgrade to WM10 despite being a huge proponent of WP8.x just to try out what it feels like. The update process went fine on my Lumia 920 which earlier had WP 8.1 Update 2. There was severe lag when it first booted up. I could not properly even open basic apps like Phone or Messaging without significant lag. I had a backup anyways so did a hard reset and then things started to settle down. Here's my partial review after playing with it for 2 days -
1. There is a perceivable but not very evident lag, in the overall experience compared to WP 8.1. The irony is that individual apps open and resume almost instantly compared to earlier versions but the 'overall' experience still has a perceived lag. It could be due to the ageing processor of the 920 as well. Mind you that this update has a lot more features than WP 8.x ever had so older hardware do tend to feel a bit slow. But its not crippling as individual app experiences are better than the predecessor. May be a little fine tuning could be required for older hardware to cope up.
2. I so much liked the square contact cards and full screen "Calling" cards in 8.x and am utterly disappointed that the circles are more of a iOS/Android wannabe. I hope MS re-considers their design decisions in the future.
3. As pointed out by others in various threads, SMS restore is not complete. I understand that the system has changed to a conversational organization but a lot of SMSs from 2014-15 are still yet to be restored.
4. Speech input doesn't work on my 920. I tried installing new language packs, changing regions etc., but of no use. Calling mic is functional. I guess new firmware is needed to properly enable speech input. Even on my Windows 10 laptop, the inbuilt mic didn't work until I had installed updated drivers. They should release new firmware/drivers for older devices if they truly intend on supporting them.
5. Glance shows only the day but not the full date despite enabling the same in the settings. Could be driver/firmware related. They should update all the system apps to support Windows 10.
6. I am impressed with the actionable notifications. They make work so much convenient. Devs must update their apps to support this (I hope they don't delay given that new phones are launching). Notification centre could use some translucent effects though.
7. Battery drain was very high. This normally happens during the first few days of a new OS install so am holding my horses here. Will study thins over the next few days.
8. People app and Messaging apps open very slow. Facebook Beta (for Windows 10) takes forever to sync with People.
9. Phone gets a lot warm during normal use.
10. Haven't played any games yet. So can't comment on gaming performance.
11. I think Groove music is finally finding its well, groove
The initial impression is far better than that of Xbox Music.
12. The vibra has stopped working. The 920 did have a peculiar problem wherein the vibra just doesn't work sometimes but if one tries to manually initiate using any of the system apps, it used to start working. That's not happening now in W10M. Again, could be driver related.
Overall, it's a worthy daily driver contender but is still not up there yet. Microsoft should finalize a suitable build candidate for devices other than the 950 series and bundle firmware updates as well along with it for the best experience. They should release it asap for Insiders and CV models so that many devices can be updated as fast as possible which would in turn provide incentive for developers to create/migrate their apps to the new OS. Would I go back to 8.1? Not right away. Would like to spend a few more days with it. But as of now, I would give it a 7/10.