I have the S7 Edge but am seriously considering switching to the M10 in August when I have an upgrade. The S7 Edge is a great phone (especially the screen and camera) but I've a a few lingering issues which I never had with my previous HTC phones:
1) the bluetooth doesn't play well with my car (2011 Audi Q5). It will connect just fine but I cannot import my phonebook. I asked at the dealership if there was a software update for the car and the rep's first statement was: "Let me guess. It's a Samsung phone". I suspect this is not widespread and perhaps isolated to certain manufacturers but, at least for me, this is a problem. Cheaper to switch phones than cars;
2) MS Exchange: maybe it's just the Samsung stock email program but I've encountered a few missing features. Nothing major, but annoying. Stock HTC email always worked perfectly for me with Exchange;
3) Do Not Disturb mode: I've played around with this at length but still cannot get it to work as I'd like it to. No idea how it is on the M10 but always worked without issue on the M8 & M9 for me;
4) Fingerprint scanner / home button: hate it.
5) Notification management: I've had the phone since release and it's still damn confusing. I've pretty much given up at this point;
6) TouchWiz selecting default apps: I have no idea what they were thinking here. You have to go deep into the settings menu to change them. I hope it's changed in a software update but nothing yet;
7)TouchWiz, in general: no lag that some people appear to be complaining of but it's still an ugly, confusing mess, in my opinion. Sense by a mile here;
8) Screen is a scratch magnet: within days of careful use there were minor surface scratches on the screen (nothing major, but visible in certain light). I didn't plan on doing so but have resigned myself to having a screen protector on the phone. And, they are a pain in the *** to install on the edge;
9) The "Edge" screen: I still don't see the point. I bought this variant for the larger screen and battery. Other than that, I never use the useless edge features.
Anyway, not sure if I'll make the switch or just wait until next year. All in all, my gripes with the S7 Edge are relatively minor and it's still a great phone. In the event that I switch, I'm fully aware of the screen and camera downgrade. As long as the M10 screen and camera are decent this isn't a big issue for me.