I've had the HTC M8 for a little over a week now, and it is awesome. I had a HTC One X (initial generation) prior, so it was a significant upgrade in computing power.
The load times are 1/4 as much as they were before if not less...this is both the loading of saved materials (apps) and when DL'ing stuff.
Screen is VERY reactive, I believe it 'set the record' for quickest reaction time of any touch screen device, there is no delay whatsoever
The only issue I have is the most complained about issue, & that is there is a 1/4" black bezel on the bottom of the phone that is visibly nothing other than the HTC logo, so it makes one handed use of reaching the top the screen 1/4" harder...HTC has stated that area is needed internally for hardware, so it was unavoidable.
I'd recommend it though, and I really expected to move to the GS5, but when I used them both side by side @ AT&T store, the S5 didn't stack up in areas referenced above