So here is a quick mini review of my Lumia 640 AT&T unlocked upgraded to WM10. I am using it on T-Mobile for the last two days. Everything seems to be working without any issues. I was able to restore my WhatsApp without any glitches so that is a big plus. Some of the new features are convenient - like replying a message through notification, swiping away individual notifications to name a couple. I decided not to install eBay and PayPal apps since eBay app is half baked and PP decided to pull support. Mobile websites for these two on Edge work just fine, I test drove them right after I setup the device. I got the phone to update to WM10 and then I updated all apps and then downloaded the next update, hard reset the device and then set it up as a fresh device. No issues running it as a daily driver. Everything works just like it did on WP8.1.1. Battery life is decent, not too shabby but it isn't terrible either, I get about 35% left after a full days use, versus between 55% and 60% on WP8.1.1. I also noticed that for some weird reason the device gets better signal on WM10 compared to WP8.1.1.
More updates as I use the device through the rest of this week.