For anything it's worth, I bought the David Clarke X11 (yep, that's the expensive Automatic Noise Reduction set) bought from Florida, it does very well at reducing the low frequency stuff like the engine noise but does not seem to touch the high frequency stuff. This may be an advantage to people who are concerned that they may not hear the stall warning. I must warn you too that my basis is only with about three hours of use compared to the (excellent) David Clarke 13.4 set.
They are extremely comfortable though, and even more comfortable than the DC 13.4's (if that's possible). As a word of warning, my instuctor (Henry) wore his DC 13.4's in preference to a Sennheiser ANR set dues to comfort (he only wore the Sennheiser set on a 3hr night cross country dual when he plugged in his iPOD).
Having said all that I will not be going back to my David Clarke 13.4 (as good as they are - especially with the cloth covers) after the David Clarke X11's - any attenuation is a good investment in your own hearing. I hope one day to still hear the whispers of my wife next to me.
Ah,
T.
PS To put it into perspective though, how on earth can a headset cost the same as a leather sofa....