I do agree that the M9 is an 'evolutionary' design. To me, the M7 was a 'revolutionary' design as it was the first android phone to incorporate the premium finish, and introduced what has never been seen on any smartphone. (BoomSound).
Those from firing stereo speakers caught everybody by surprise. Then, The M8 took it a step farther and improved it's build quality, louder 'boomsound', and it was as premium looking as you can get for 2014 (android wise). They addressed the external storage option with the M8, but kept the same ultrapixel technology that was a hit or miss.
Then, the M9 addressed complaints with power button placement as well as camera. They also changed the assembly on the edges to improve grip as the M8 was very slippery to hold.
The camera comparisons between the M9 and S6 remain to be seen. But Samsung took 2 years to re-design the look and feel of the S series (well, just the sides and the back, as the front of the phone looks pretty much the same)..
To me, the M9 is just making what was already great with the M7 and made it probably almost perfect.
I still have my M8 and probably won't get anything in the near future as my contract has another year +.
Then, i will have to consider what is out for 2016. If the Samsung keeps "back glass", i don't know if i want to go that route. Also, TouchWiz needs to go in my opinion. The simplicity and fluidity of HTC Sense is one of the factors that makes me love my M8 more and more.