I don't have an HTC One M9, I haven't even held one -- I did have an HTC One M8 though, which is sufficient enough to pen a few thoughts about the new M9. It's practically the same device, when the display doesn't change the the camera is still abysmal, what more is there to really say?
HTC is trying to take the Apple approach to their line, which is working to an extent. They have excellent hardware, but they have their shortcomings. The camera is still atrocious; they've now needed to re-focus from the 'Ultrapixel' gimmick and they're struggling with the transition.
Many believe software updates will fix the poor performance, but it seems to me to be more of a hardware issue than anything -- they went cheap on the glass and they're paying the price.
Everyone likes taking photos of nothing in 2015, this isn't the age where you're able to pass with an abysmal camera unless you're BlackBerry because your user base has no desire for media first mentality.
It's sad that they skimped because the hardware is amazing, it feels great in hand and the speakers can't be touched. But with Samsung and LG having 2k displays, you can notice the difference in the battle of the juggernauts. My Galaxy S6 makes me hate using my iPhone 6 for example.
When there is something better available, it'll always make the shortcomings of the competition more prevalent than they actually are. There is no need for a 2k display, but that doesn't mean you don't want one.
If you have an M8, I don't see a reason to 'upgrade' to the M9 -- if you're due for an upgrade, I'd look at the S6 first and then draw a conclusion based off that. It's a great device, but it isn't the flagship that it could have been. This is the second 's' release, by Apple terminology, which means this revision should've been a bit more extreme than it actually was.
Device refreshing can only go on for so long before something seems dull.