Yes, I have... although I do still have the M7!
I don't really have the need for a 'high end' model, as I also have a OnePlus One on Vodafone, so I ported my number over from the HTC One M7 to a Microsoft Lumia 640 on EE. I went with EE as I already have one 'line' with them so got a second one for £14.99 a month with 500 minutes which is more than enough for me. Their WiFi Calling feature was something else which appealed, an it will certainly do me until Windows Mobile 10 has settled down, and some more phones have been released!
If you are used to using Windows 8.1 on your desktop PC, then the Mobile version shouldn't give you any problems at all... and I was able to sync in my gmail account and contacts also, which is a bonus.
As I mentioned in that section of this Forum, I am more impressed with the Lumia 640 than I thought I would be, as I like the simple styling and find a 5" screen to be a good size.... looking at the "Images of Items I have Purchased" thread it appears that a few other members have gone for this model from EE also.