I have used the UFlyMike A100 with the QC2 and QC15s for over ten years and decided to upgrade to the Harmony System and the QC25s. I finally received my preorder last week and used them this week for a quick domestic flight from KMMU to KSRQ and back in a G550.
Here is my quick review:
Bose QC25s: I did not notice a substantial difference in noise cancelation and sound reproduction between the QC25 and QC15 headphones. The most significant and important difference is that you still hear audio if/when the battery dies. This is a major improvement over the QC2/15s.
Harmony Boom Mic: The boom is much thinner and way more flexible than the A100. You can order it in three different lengths...Standard, Long, and Extra Long. I ordered Long which is six inches in length and the mic reached easily to the center of my mouth. The mic is also smaller and has a small white dot at its base, which makes it easy to discern in which end to speak.
Aviation Application Cable: I ordered to two plug cable. It features a rotating gain adjustment wheel, mono/stereo selector, L/R volume control, two female jacks (one for and Aux Cable and one for the boom mic), and lastly a microphone output selector to either the Aux or boom mic.
The G550 is already pretty quiet so I started with the Gain at 1 but moved it to 2 after about 45 minutes of flight. In the G550 you can adjust the VOX (sidetone), which I set to 9. With this setup, the voice activated intercom did not come on unless I spoke or exhaled forcefully. ATC heard all of my transmissions loud and clear as did my fellow crew member over the ICS.
Often times we fly out of Uncontrolled Airports that require us to call ATC on the phone for clearance. For this, I connected the supplied AHJ/CTIA male to male Aux Cable to my iPhone 6. I moved the mic output switch to the phone position and made a trial call. The person I called said my voice was muffled and was "in and out". As it turns out, the iPhone 6 has a known issue with third party mic cables. See this article:
A word on the iPhone 6/6 Plus and third-party headphones | The Wirecutter
With my cable, one end worked fine and the other end (the end I made my trial call with) needed to be pulled out ever so slightly from my iPhone 6 to work properly. Problem solved.
Carrying Case: Larger than the Bose QC25 case but not too big. It fits easily into my eBag Professional Slim Laptop Backpack. It also holds everything you need, Mic boom and cable, Aviation Application Cable, QC25s, Aux Cable and a spare AAA battery.
All-in-all, I think the Harmony is a great improvement over the A100. It is more flexible, smaller, and customizable. You can use your phone with it to call ATC or dispatch without having to take off your headset. The Bose QC25s work even if the battery dies.
For these reasons, I highly recommend the UFlyMike Harmony.