I'm thoroughly enjoying W10M on my 640. I've not had battery issues. I got in just before Official hit and have kept on Release Ring since getting patches and fixes right along.
Anniversary Update is coming very soon and it is going to have even more features.
Right now I think the #1 advantage is deeper integration across devices. Stuff is syncing up nicely between phone and PC. Alerts/Notifications/Alarms/Reminders everywhere. Get Text notification on your laptop or PC and reply from your desktop. Or initiate and send a text from PC with Cortana.
In Phone backup you can save App data. Improved UWP access.
Camera is more powerful on the budget devices. Forced HDR on your Lumia 640.
And check this trick out I just got rolling with today. If you have W10M you can use a tool to edit your registry and get DNG capture on your device:
http://forums.windowscentral.com/gui...our-phone.html
I'm digging W10M. YMMV