I've got some first hand experience to report on the UFlyMike/QuietComfort2 combo vs. the Bose X:
I've had the Bose X for 5 years. Lightweight, great customer service, awesome noise cancelling. They've served me well from the days of Cessnas to Brasilias and then onto long legs in the CRJ.
I saw the ad for the UFlyMike adapter in the SFO crew lounge and it about stopped me in my tracks: half the price ($200 for the mike boom adapter and $300 for the Bose QC2) and the ability to route an audio line directly into the headset. Plus, an awesome quality headset with noise cancelling to listen to music while deadheading, or just generally rocking out. I figured if I could sell my old Bose X for even half the price I bought them for it'd would be a wash financially and I'd have a new headset with the bonus of the audio input.
I spoke with the really nice people at UFlyMike (Southwest pilot husband designed the product and wife handles all the sales and marketing) and the lady told me if I was used to the Bose X to expect about 80% of the noise cancelling but a lighter headset overall.
I got the Bose QC2 in the mail and was blown away by the audio quality, especially the clarity of the bass with no distortion. I was very excited. One strange thing I noticed was when I turned on the QC2 noise cancelling I did not get the same level of quiet I would if I wore my Bose X just sitting in a room. I figured I had to try them out in flight. The UFlyMike adapter came and I set everything up and I was ready to rock, er... I mean, "Go to work."
Long story short (well, not really): after one round trip with the new headset I was ready to send it back. The UFlyMike/QC2 combo had a really noticable high-pitch hiss in the background, but my main issue was that the noise cancelling was nowhere near that of the Bose X. I was told 80%, I would say it would be generous to rate it 60% of the Bose X's performance. I think the problem there stems from the fact that the QC2s were designed for excellent bass production, and the sound quality while hooked up to my mp3 was second to none, but this in many ways defeats the whole purpose of noise cancelling, as the worst range of background noise is in that lower level. The Bose X was designed specifically to mute out and cancel those low end frequencies and does a great job at it. Anyone who has attempted to hook up the Bose X to their home stereo can attest to this: the bass is almost non-existant (bad for stereo headphones, great for aviation).
Anyway, maybe I've just been spoiled all these years with the Bose X. I've had them for so long and have gotten so used to them that I really have a hard time trading down to any degree. Is it worth the extra $500? Maybe not if I had never tried them before, but as they say, "Once you go Bose..."