I quote Anandtech's review:
The display is one of the more disappointing aspects of the One M9 thus far. Although the 1080p resolution isn’t really a problem in actual use, the lack of improvement or regression in every other metric suggests that HTC has gone backwards in display quality. Combined with the removal of PSR, it’s concerning to see that cost optimization has affected such a crucial aspect of the smartphone experience. Overall, even when comparing against 2014 smartphones the One M9 ends up closer to the bottom for display quality.
The software experience seems to be one area where HTC continues to do relatively well compared to most Android OEMs, and Sense 7 remains a relatively enjoyable experience. However, the new additions to Sense 7 aren’t really all that helpful. There’s a lot of effort spent on introducing new features like additional personalization and some new widgets and applications, but none of this really feels well-differentiated, and there isn’t much change to the rest of the UI to make it mesh with Lollipop’s UI. It also seems that Snapdragon 810 causes some minor performance issues, but the effects of this are incredibly minor and it’s hard to tell whether this is due to random variance to some extent.
Although we’re still missing some of the pieces, based upon what data we have the One M9 is in an alarming place for a new smartphone. It’s pretty rare that a new phone ends up regressing in almost every major way compared to an old phone, but the One M9 ends up doing this in display and battery life. The SoC is better, but I can’t help but feel that Snapdragon 805 ends up being a better choice than Snapdragon 810 for a flagship smartphone at this time. The Snapdragon 808 may be better suited as an upgrade to the Snapdragon 805, but given the performance of the 810 I’m not really holding my breath. Given all of these issues, I’m almost tempted to point to the One M8 as the better phone, but until we get the full picture we won’t be coming to any final conclusions about the One M9.