HTC has been struggling in recent times due to the poor sales that their recent devices have achieved. They are definitely not bad devices, they are often considered among the best-designed ones and they integrate good specs overall. Perhaps one of the weaknesses of the devices is in the camera department, as the ones that they have included, are not bad, but let's just say that consumers now have very high expectations from the cameras on flagships. The company has opted to include in their offerings some exclusive technology like the BoomSound front-facing speakers or even some extra services like their Uh-Oh protection plan against accidents, which are certainly interesting.
On the software side, the Sense UI has always been a very good customization of the Android operating system. It features a very good design in the whole interface and it has some extra features like BlinkFeed, plus it allows users to personalize the way it looks with various Themes. HTC has always shown commitment to update their devices to the most recent version of Android, in fact, with Lollipop, they promised to get their devices updated within 90 days after Google released the code so that they could customize the OS. They weren't able to keep that ambitious promise, but it wasn't too far away when they updated their devices either.
Now, Google has just announced that their Nexus devices will start getting Android Marshmallow on October 5th and HTC has now announced the devices that will start getting this update, which will probably come with a new version of Sense. It's mostly the same list that got leaked a few days ago, but the former flagship was added and the Butterfly 2 was removed. Additionally, some smartphones like the One M9 and the One (M8) will start getting the update at the end of 2015, which is quite fast. Other flagship devices that will get the update a little later are the One M9+, the One ME and the One M8 Eye. Some of their mid-range offerings that are confirmed to get the update are the One E9, One E9+, One E8 and the Butterfly 3. Some phones of their Desire series are also getting the update, including the Desire 826, Desire 820 and the Desire 816. More devices could also get Marshmallow, but they will be announced a little later. Let's remember that the updates also depend on carrier's approval, so it might take a little longer before those locked devices get updated.