From an audio quality point of view these headphones are a lot better than the Bose ones. (I often listen to Alan Holdsworthian jazz, so I am always *very* analytical about the audio quality) and another strong point: There is no Micro$oft installed, so no updates or other things engineers really don't like in their spare time ;-)
I use these noise-cancellation ear-buds on longhaul flights to quieten the plane noise, usually with no audio running, which makes me less fatigued. They also sound pretty good with audio.
Max, actually in-ear monitors (IEMs) have a noise isolation capability of up to over 40 dB compared to the mid 20-dB range for active noise cancelling headphones, which also usually only work for constant noise and over certain frequencies.
On the other hand, IEMs aren't for everyone - some find them uncomfortable, and cable microphonics can sometimes be an annoyance. Also, some (not all), like the Etymotics, stick out of the ear far enough that you can't really sleep on your side in them.
I think that you have to weigh up the relative cost. Relative, say, to a cabin upgrade (which might not be quieter anyway). Then divide by the number of trips/flights/hours (I made around 12 round-trips in the last year, totalling about 40 individual flights), and the numbers start looking quite small.
I was chatting to a guy in the office who said that he found a few hours of airplane noise to be very debilitating -- it left him tired and fractious.
I always travel in the cheapest coach seat money can buy (if you see what I mean) so anything I can do it improve the quality of the trip counts.
This was a one-off for me. I'd just been paid for a project and had covered all of my bills and decided to give myself a treat.
I have spent countless hours on planes -- typically I just read books and (as you say) get used to the noise. It was having my iPad that led me to start watching films and suchlike.
For my upcoming trip to the UK I have all the episodes from Dr. Who Series 6 Part 2 loaded on my iPad. On the one hand I do feel guilty about splashing out thsi cash for the headphones ... on the other hand I'm telling myself that after 30 years of suffering the aircraft noise it's time to "smell the coffee" and "hear the bird songs" :-)