I got the notification for Win10 Mobile 10536.1004 also. In USA on Verizon, update went well, only took about 30 minutes, coming from 8.1.1.
After playing around a bit, updating apps, etc. noticed that a few apps (MS Maps) didn't update. Then noticed that I had no WiFi. It was disabled, and could not turn it back on, even after soft reset. Now performing a hard reset. May try again, if option is available again. Did like most of the style, features and feel of this version (10536.1004). Was really shocked to get the notification though, as no one seems to admit that it is available for anything but, MS & Nokia Lumias and the HTC One(M8).