I had an opportunity to fly with a Bose A20 headset yesterday and thought I'd share a bit about that experience. First of all, the active noise cancellation is good, very, very good. It takes a whole chunk of noise away, all in the lower frequencies. There is an interesting caveat though: as the passive noise attenuation of the headset isn't as good as my regular headset (DC H10-13X) you appear to have more higher frequency noise left. In comparison, the DC quiets all frequencies before taking out a chunk of the low frequency rumble with the ANR. I was flying a Rotax-powered AT-3 that has quite some interesting high frequency rattles and noises throughout the airframe and these appeared to be more pronounced with the Bose, while the DC did a better job of quieting the whole noise experience. This may of course depend on the type you're flying.
If you're used to passive headsets, a Bose will provide you with a noise profile that is quite a bit different from what you're used to while a DC or similar will most likely appear somewhat similar. I was still very much impressed by the Bose's performance, don't get me wrong, but it would take me a while to get used to the noise profile as I've flown for so long using DCs or similar types.