Temptation was too great, so picked up a S5 yesterday after a brief stint with the M8.
Overall I'm more satisfied so far with the S5 than the M8. The M8's materials and unibody construction did feel good in hand, but the power button placement required too much hand shimmy to reach. There were some QC issues that bugged me such as the speaker grill not flush with the screen, and playing music would cause random skips/pops. Sense 6.0 is very smooth so props to HTC. Samsung's new Touchwiz is better but nowhere near the optimization that Sense 6.0 brings.
The S5 despite it's plastic build is pretty solid, and because of the grippy texture on the back felt more secure in hand. Replaceable batteries is always a plus but that stupid charging flap will drive me nuts, so I will order the Qi compatible back once Samsung decides to release it so I can use my wireless chargers.
I was thoroughly surprised when setting up iCloud through the native email client on TW, and it gave me the option of Push. Has that always been there? Don't remember Samsung supporting Push email of iCloud when I had my S4.