Compared to my previous headset, DC 13.4X (ANR). Audio quality was crisp and clear with the DCs, made you sound like a badass on the radios ha. The mic on the DCs was excellent, it would come on quickly when spoken into, and stay activated. It didn't squelch easily if air was blown onto it from vents etc.
Audio quality, mostly hearing your own speech back is poor in the Sierras. The mic is fairly sensitive to vent air etc, and your squelch must be set precisely in order to balance this with it actually staying keyed while speaking into it. I believe there are some sensitivity adjustments, but I haven't messed with them much (yet). The DCs were great out of the box.
The Sierras are pretty comfortable, still not great on the longer trips. The aux in and bluetooth features are nice, the audio playback quality is good and the volume reduction during comms is nice, it doesn't completely cut it, just reduces volume to 20%.
Said and done, I'd recommend spending more for the Bose (0% financing!) or the Zulus, both of which I've heard great things about. I wish I'd spent a few hundred more.