I have had the bose series X for about four months now. I use them flying the be1900D on the line about 85 hours per month, at 6 hours per day.
THE PROS:
Absolutely the quietest, most comfortable headset out there. I have used the Denali, and the lightspeed QFR cross country in my search for the best headset. There is not a published NRR rating on the bose, but the QFR is about 45 dB and the bose is at least that. Bose is only 11.5 oz, among the lightest headsets out there, except for the QFR. The bose headband is hinged at the top, in the middle and it works great for comfort. Its almost like wearing a set of walkman headphones. After a 6 or even 8 hour day, when I drive home in my ealge talon, (a pretty quiet car) I think , **CENSORED****CENSORED****CENSORED****CENSORED**! this car is loud! I can hear the relay for the a/c compressor click under the dash going down the road!
CONS:
the price! at 995$ is by far the most expensive headset out there! I bought mine in a retail store in St Augustine FL, and applied for the bose credit card. 12 months no interest no payment. Lack of auto shutoff feature. Battery life. 9v alkaline battery lasts 25 hours (it depends on how loud the airplane is, and how much the headset 'works"). I rotate out three 9v Li-ion rechargeables and get about 10 hours per charge. Lack of passive noise reduction. if the battery goes out inflight it is just as loud as no headset at all. The mic has to be right almost in your mouth to speak.
THE VERDICT:
I would have bought the QFR cross-country except the thing hurt my head. QFR offers no gel ear seals and it squeezes my head. plus it felt flimsy and cheap. but the noise reduction is about the same, QFR also has very good passive reduction. At about 285$ its a good value. I wouldnt give up my bose headset for anything out there on the market today. they have a 30 day evaluation and if youy try it you'll be sold. I was.