Pros: Small, compact, foldable, carrying case
Cons: Uncomfortable headsets, noisy, disappointing bass response, overpriced, very large, inconvenient battery pack (2XAAA)
The bottom line: If you can get the Centrios (at half the price), I think it would be a MUCH better choice.
I tried the Sennheiser PXC 250 Consumer Headphones and compared them with the Centrios Active Noise Cancelling Headphones. With the Sennheiser, I found lots of background noise, and unless I was VERY careful, I got a lot of crackling noise. The noise cancelling only works at low frequencies. When the batteries die (or the noise-cancelling switch is off) the audio quality is not very good. With the power swith on, I found that rather than the exceptional low frequency response I had expected, there was significant distortion.

In comparison, the Centrios provided much better noise cancellation (but again at only low frequencies). The bass sound quality was MUCH better, though the specifications weren't as good. The audio quality with the battery removed was better than the PXC-250. The separate battery pack with the PXC-250 requires two AAA batteries, and is quite bulky. In contrast, the Centrios requires only one AAA battery and is smaller. However, I would definitely prefer having the batter(y)(ies) mounted in the headset (like the Phillips or the Bose).

My testing was done in our family room with the TV on pretty loud, the bass on the TV turned to maximum, and the Laundry Dryer turned on. My favourite test CD (especially for bass) is Saint-Saens Symphony 3, Op. 78 "Organ Symphony" in C Minor. The Centrios sounded much better.

The most important plus for the Centrios was that the headphones COMPLETELY cover the ears, and are MUCH more comfortable (I think that the Bose would be at least as good here, though have not tried them). The negative side is that they are not foldable, and are MUCH bulkier for airline travel. The Centrios includes adapters for airline and for 1/4" plugs. The negative is the external battery pack which takes one AAA battery -- but at least it is smaller and lighter than the Sennheiser. Also, it has its own volume control, which is nice.

It would be interesting to compare the BOSE headphones, which look very comfortable, but are terribly expensive, and have had lots of negative reviews (sound quality, durability, etc.)

I don't know where you can get the Centrios in the U.S. -- In Canada, the only source is One Source by Circuit CIty (formerly Radio Shack).

By the way, the absolutely BEST bargain and quality in "Atomic Clock" men's wristwatches (by NEXXTECH) is from One Source by Circuit CIty: formerly Radio Shack (same manufacturer as Centrios). These watches pick up a radio signal from the nearest site broadcasting the exact time from a Caesium clock and update your watch every night about midnight. FINALLY I have a watch that gives the correct time to the millisecond ALWAYS.