Only been using it a couple days now but the HTC 10 seems like a great upgrade. I skipped out on the M9 because I didn't think it offered much more than the M8, plus my device was still going strong. Can barely notice a difference in hand compared to the HTC One M8 but the screen is larger.They have also brought back capacitive buttons adding more screen room. Finally having a fingerprint sensor is nice. Speakers aren't as loud as the M8 but def sound better. Phone runs super smooth as all my previous HTC devices have. HTC has cut down on a lot of the bloatware and its UI is pretty close to pure Android which I love. Phone feels really nice in the hand. Buttons are great. Cameras seem excellent. Battery life gets me through a day with heavy use. HTC has thrown in some nice options to personalize your device. Really digging the new Freestyle themes. Not much else. Works smooth, fast and feels great. I've used 3 different HTC devices in the past and I love how durable they are. I don't use a case and have never had any issues when the phones dropped besides for a scratch or two. But HTC devices do come with a one time replace the screen or water damage device, so that's nice.
The Nexus 6p is my favorite device on the market but the HTC 10 is just a better size for me and comes in at a close second.