I got my M8 today and I absolutely LOVE it.
The 2 reasons I gave up my original HTC One last year was because the camera was too sub-par and the battery life wasn't good.
Thankfully, the camera is much improved, particularly after applying the uFocus bokeh effect. And based on the reviews and what owners of the phone are saying, I'm expecting the battery life to be excellent as almost everyone is saying it has the same endurance of the Note 3, which is VERY impressive.
Doing everything on the phone is super smooth, the speakers are RIDICULOUSLY loud and detailed, the new gestures work seamlessly, the front camera is extremely impressive (As you would expect being a 5MP shooter), Sense 6 is very clean and functional (definitely the best OEM skin by far), and the design/build quality is outstanding (even better than the original M7).
All in all, HTC has another winner on it's hands, and most likely the best overall smartphone of 2014 (We'll have to see what Apple brings out with the iPhone 6, and Samsung with the Note 4 though). And while everyone called the M7 a flop, I don't think 6 million units sold is a bad thing at all.