xpd:
I'm starting to look at jumping ship (again) back to Android, I'm finding with the few updates lately on my 640, that its getting slower, apps crashing at random and just generally app support is really starting to lack :( Even my wifes Galaxy S3 is more responsive than the 640 now....
I'll miss the "single platform" for everything (my phone, Xbox and PC all play nicely together) but unless something major gives WM10 a push, Ill be moving on...
I found with Android I had to "babysit" it all the time, at that syncing with Exchange servers were just painful. It's been a while since I touched Android, but I'm not in a hurry to see if it's improved.
Having said that, since last two updates to Windows 10 Mobile I'm finding the same thing happening with syncing Exchange accounts. (Keep getting a "We couldn't obtain the single sign in token to authenticate" message)