Bought the Halos about two months back but managed to only put in 10 hours on them so far - and I'm fairly pleased. I couldn't tell you whether they are quieter than a pair of Bose A20s (which implies they're quite close in performance).
A couple of things to note though - you wanna have clean hands since the inserts you mold with your fingers go into your ears, so do your preflight wearing gloves (I used to do this before I got the Halos anyway) or wash your hands before flying.
Also, they require a few seconds longer to 'put on' - unlike the over-ear headsets which you just put on, with these you need to put it on, mold the foam insert, reach over with your other hand and pull on your ear to straighten the ear canal (though not necessary from my experience) and shove it in. After a few missed approaches, I found it takes me about 10-15 seconds to get it all set up at the beginning of a flight.
If you are doing a bunch of small flights or any other type mission where you constantly have to take the headset on and off - this might be a nuisance. The silicone/rubber inserts (instead of foam ones) should be quicker to insert since they don't require you to mold them with your fingers before inserting. Some said the silicone inserts don't provide as good noise cancelling though.
The pressure concern is unfounded, since the foam inserts do allow air to pass through in small quantities. If you were being shot out of a cannon vertically up to 10.000ft you might feel a difference wearing them, but the pressure gradient we experience while climbing in an aircraft isn't steep enough to feel any difference. The inserts do NOT seal the ear completely.
Another thing worth mentioning is it's design - it's almost nothing but wires and tubes, so you can comfortably leave it on an aircraft seat and accidentally sit on it without any risk of breaking it.

There's nothing to break, really - wire might bend but you bend it during normal usage to fit the shape of your head anyway.
And finally, the guy who makes them is a great guy who had patience with me asking him a bunch of questions, and answering them all - of course.
In case someone else needs a link:
www.quiettechnologies.com