Just bought one from Transair in December as they were doing a promotion with free flight bag.
I have been using them at work and the difference between these over ear, and our a/c on ear Sennheiser ones is incredible.
Ive yet to try it in a GA aircraft as yet but will do so in February.
The one thing I would say is the ambient noise is impressively reduced but also reduces the ability to identify any sounds that would be identified as impending problems unless you have EICAS system or a bloody good ear/feel for the aircraft.
I suggest if you can afford it buy new and take advantage of the 5 year warranty that comes with it as opposed to buying second hand at near 75% of original price and whatever is left of said warranty.
But like most things aviation.
Budget dictates