I have tried 6.5 on the x1 and its gooooood!!! Obviously the occassional bug as can be expected from an unfinished platford but well worth the change!

It's available at xda-developers.
It is reversable, all you do is reflash with the ROM of your choice - there are plenty of custom as well as official roms out there so lots to choose from!!
WinMo6.5 is not expected to be officially released until Late Q3/Mid-Q4 2009 so expect it around September - November this year but why wait??! The 6.5 ROM is so easy to use and there are always additional hacks and tweaks being added to it.
For all of the above, just look at the xda-dev site!
Hope this helps,
Nirave