Sennheiser PXC 350 Noise Canceling Headphones - Reviews
Pros: The best sounding NC headphone I have heard; also sounds good with the NC off
Cons: positioning on the head critical to having the NC work well
I bought a pair of PXC 450's when they first came out.  I travel a lot for work, and I wanted the very best NC headphones I could get, since I have never been able to like IEM's from a comfort perspective.
Unfortunately, the PXC-450's were fatally flawed - when the NC circuit was engaged, the bass went completely away.  I mean completely.  So I returned them, disappointed.  I was using the Audio Technica ANC-7, which are very good, but I was certain there was some additional performance to be had.
At that time the PXC-350 were not available.  Finally I spotted a pair at one of those airport kiosks, and I bought a pair on a whim.  Fortunately, I have really enjoyed them, and have been using them regularly ever since.
The PXC-350 sound very good, both in NC and non-NC mode.  They do sound different in the two modes.  While the 350's have a little less bass in NC mode than non NC mode, it's not significant.  The bass is probably better in NC mode, actually.  Same with the treble - a little cleaner and better defined with the NC circuit ON.  But either way, they are a good pair of headphones in terms of sound.  The sonics are good enough that I almost never travel with a second pair of headphones anymore.
In terms of the noise-canceling, it is generally very effective.  However, the NC efficacy requires the earcups to be tightly coupled to the head.  The issue there is that the earcups are large, and this means that the seal works fine when you are facing straight ahead.  But turn your head too far to one side, or if you are chewing, the seal breaks, and the NC circuit is much less effective.  This can be a little annoying.  I felt the overall sound quality makes this compromise acceptable, but it's the one annoying aspect of the PXC-350's performance.
Battery life is very good - bordering on exceptional.  Overall, the PXC-350 is a very good headphone - and at the current street price of more like $250, well worth it.