I was pretty happy with the M7 and I upgraded to the M8 around five days ago and have never looked back. I gave my M7 to my Mom and I must say that I am LOVING the M8. One of the biggest advantages for me is the biggest screen on the M8; although the difference between 4.7" and 5.0" isn't that huge (especially considering phablets like the S5 or the Note), it still makes a huge difference to me not only because of the size itself but also because of the clarity. What differentiates HTC than any other Android phone to me is not only build quality but also screen clarity and colors, which are all just stunning.
The camera is a huge improvement as well because I had the purple tint on my M7's camera and it was extremely annoying and the dealership wanted a lot of money to fix it.
Another improvement is obviously the speed and multi-tasking - although the M7 does an amazing job considering it is a nearly 2-year old phone when it comes to performance, multi-tasking, gaming, and such, the M8 is just that much snappier and faster and I immediately noticed it out of the box.
And then comes one of the most important things for me - battery life. The battery life on the M8 is much better than it was on the M7 for me, and that's a huge advantage as I don't always have the option of charging the device when I am outside (which is quite frequently).
And last but not least BoomSound is actually pretty good and I have not regretted it although a lot of reviews said that Beats Audio was better (I honestly don't see it that way at all, and trust me I am not biased I LOVE my M7).
If you aren't that excited for the M8 and you don't see that it's a huge leap from the M7 then don't upgrade and wait for a newer HTC device to come out(M8 Prime? M9? Who knows).