"I was underwhelmed with the M9. I've had it since April, and it's been laggy, stuttering when opening apps or switching apps, almost as bad as a Samsung device. Battery life is easily the worst in any phone that I've had since the Nexus 4, especially standby time. I replaced it with a Sony Xperia Z3+, which is the total opposite. Very fast and smooth, no lag anywhere, and the standby battery life is so much better. I can leave it off the charger overnight and it might drop 1-2%, total.
Also, most of the pictures I took with the M9 were just terrible. No matter how hard I tried, they look grainy and blurry"
I have not had that experience at all. My HTC One M9 is very fast. As a small example, it's the only phone across 4 different phones I've tried that I've ever been able to play Flappy Bird with no lag (yes I play Flappy Bird). I experience no lags at all opening or running any apps. Battery life seems comparable to the Galaxy S6 Edge that I had for 6 weeks previous to the M9, but of course I didn't do any benchmark tests between the two. The day time pictures are great, but yes I'll concede that the night time shots are very challenged.