If it only cost you around an "upgrade cost" it's worth upgrading. If you had to pay no contract price no it's not worth it.
My HTC One M7 has the original wide dynamic range dual mics. It's a very overlooked feature that just rocks.
Could care less about camera gimmicks.
Camera seems about the same. And it's fine.
I do like the SD card.
I still prefer the size of the M7, it was perfect. And it's weight.
Sound wise it sounds the same. And that's with beats off on the HTC One M7. With beats on I suspect M7 sounds better.
I prefer headphone jack on top.
Sense 6 is no biggy, but it is a tad nicer.
My wife has the HTC One M7 now. The clarity on phone calls between them is extremely good.
M8 does seem to pull LTE better and maybe transition better between 3G and LTE better (maybe). Does some funky things (Spark things I guess).
It's not a huge upgrade for sure.
Samsung Galaxy S5 with R.O.D.T