While I have been happy with my 8x, I'm in the buy Microsoft only phone from now on (outside of a cheap LG Lancet replacement if something happens to my 8x between now and W10 flagship in Verizon).
While it might become more of a moot point since Microsoft has announced they are going to be in charge of OS updates, not the carriers, there is still the chance that an update might require a firmware update at which point the phone may very well be screwed.
As far as the 8x getting W10, I really have no clue as it will be close to 3 years old and be 2 generations of devices behind when W10 gets release, so while it could technically have it, if there's a firmware update required, I think it will be highly unlikely that it will see it.