I own a Lumia 640 and would want to upgrade to W10. I've heard positive reviews about L640 running W10. So should I upgrade through IP or wait for official update? Also will my apps data get lost during the upgrade and is there a need for a hard reset?
This is a good time to test W10M. If you don't like it, there is still a way back to 8.1 (automatic update checks can be disabled after that). Upgrade should not kill any data, Health has changed to something else so not sure about steps etc.
Hard reset ("Have you tried turning it off and on again" or "reinstall W95 - again" revisited
) and revert to 8.1 will wipe all data not stored to cloud. E.g. Sports Tracker and Geocaching Live have their own servers. Some games save to One Drive or FB (?). Some data will be lost for good. It's the same with Android.
It's really a matter of having nothing better to do and possibility of losing game stats or WhatsApp conversations (it has to be backed up to SD - guess did I know that on first time) in hard reset, if/when upgrade process does not get app and setting migrations done the way it should in final, official, update.
There is no guarantee that the official upgrade will be any better. I hope it will be, with firmware and stuff, but I'm not a lottery winner type so --