When turned on but not plugged into an audio source, the Sennheiser headphones reduce white noise, such as jet engines and road noise, by about half (best estimate). It is enough to provide some relief but it does not reduce sounds like talking or clattering, so you will be disappointed if your goal is complete sensory deprivation.

The competing Bose headphones that I tried in stores provide better isolation but cost more than twice as much, are less compact and block air flow around the ears. The Sennheiser headphones allow ears to breath better and are comfortable enough to permit resting on a headrest or pillow while traveling in an airplane or automobile.

The headphones fold and store in a 4" x 7" x 1" case with a main pocket and front pocket that holds the converter jacks for old-style large audio jacks and airplane jacks with two prongs. I have enough room in the case to carry the extra battery pack from a digital camera and a USB flash drive.

When the noise-canceling feature is combined with an audio source, the sound is enhanced and is noticeably superior to regular headphones (especially the junk passed out on airplanes). I use them only once a month so their cost seems high but I am glad to have them when I am trying to hear or tune out sounds on a long flight.

The two AAA batteries lasted about ten hours before showing signs of fading. The battery pack is in the middle of the cord and can be clipped on a belt or kept in a pocket, leaving enough cord on either end to reach the audio jack and the headphones. Often, however, I find myself wishing the battery pack was at the plug end of the cord so I would not have to juggle it in my lap or lay it on the seat next to me.

I concluded there were no perfect headphones when I was shopping and opted for the Sennheiser's features and price. I may have chosen differently under different circumstances.

FYI: The headphones I bought in July 2004 were made in Ireland. Lately, I have seen on packages in stores that production was moved to...guess where...no, not Switzerland, keep going east... Chinese quality seems to be improving but they should have passed some of the cost savings along to the customer for a product that has been out for quite a while.

If you fly a lot and love music, this is your headphone. With each flight, I am constantly amazed how the outside world disappears the moment I switch on this device. It doesn't cancel all noise (ignore the amusing review that pitted these headphones against a vacuum cleaner), but it does drown out enough background noise that you will seldom hear people talking around you (ie: cellphone yakkers). Overhead "please return to your seat" announcements cease to exist. In my non-scientific estimation, airplane engine noise is reduced by 80%. It's unreal. Buy a pair, take a flight, turn on your favorite song, listen to it for a few seconds without the noise-canceling feature, flick the switch and enjoy. You'll be tempted to share the experience with your seatmate by the end of the flight. Disclaimer: I have used these headphones exclusively on airplanes. I do not know how well they perform on a noisy street. I suspect, however, that they'd be perfect for train/subway/bus commutes.

For all air travelers: I would ignore the comments of all users who didn't give this pair of headphones a chance on an airplane.

No one can appreciate the value of these headphones until you bring it with you onto a jumbo jet. You have no idea how loud airplane noise really is until you've tried a pair of noise cancelling headphones. When I first tried these on at Costco, I was impressed with how it drowned out the ambient noise of a busy warehouse store. However, on my trip to Sweden last fall, when the plane started its engines, I kept on clicking on and off the noise-cancelling switch (like Tom Hanks in Cast Away with the hotel room light) with my mouth hung open in amazement.

The difference was dazzling. The Harry Potter movie was showing, and I could hear the wind noises from the movie. With the noise cancelling off, I could barely hear the dialog. I was so excited that when someone asked me about my curious headphones with a separate wand clipped to my belt, I immediately plopped the headphones on her ears and watched her jaw drop in amazement as a clicked the switch on and off.

These headphones are moderately helpful with drowning out subway and road noise. I think vacuum cleaner noise might be too high frequency for the noise cancelling to be as effective as on the plane.

For the price, and for its effectiveness on an airplane, I think these are worth their price for even one air trip.

I've been using these for almost a year, and if you're looking for a small, portable, foldable, lightweight, travel-friendly set of noise cancelling headphones, you can't beat them.
Since the earpieces do not completely enclose your ears, there will be some amount of ambient sounds coming through, but for spending hours and hours on an airplane, they're the best value out there. Sure, some others may sound better, and and some will enclose your ears, and still be "foldable", but for 3x the price. Let's see Bose come up with a pair that can fold down to this size!
I won't even consider going on an airplane for anything longer than an hour flight without them. For coast-to-coast or international flights (particularly the 'red-eye's) they're the FIRST thing in my bag after the toothbrush/paste.
I can carry them in my laptop bag with only a minimal amount of space, and I always carry an extra set of batteries, just in case. So far, I've only replaced the batteries twice, in about a year.
Bottom lines:
Will it cancel out all frequencies? No, but if you're on an airplane all the time, it cancels out the ones you need cancelled: the constant drone of the engines, the air conditioning, etc.
Are they comfortable? Yes, I can easily sleep with them on during overnight flights, and as they weigh practically nothing, I don't feel like I've got a brick on each ear.
How's the sound? Sounds good to me, but then I'm not an audio engineer. I will agree with some others that I hear much more in the music I listen to when I use them at home or at work. As far as the music and movies on the airplanes, there's no comparison to the ratty headphones the airlines dish out.
Will you still hear things? Of course, as I said, some ambient sounds will always come through, so if you're looking for total aural isolation, get yourself an isolation chamber. If you're looking for over-the-ear studio-type NC headphones, they'll probably be much to big to slide into a laptop briefcase, or cost at least twice as much (or both).
So... If you're looking for some very good lightweight, travelling headphones to cancel out the incessant whine of the engines, (and most of the incessant whine of the 4-year old two rows behind you), these are the ones to get.
I've recommended them to friends and co-workers alike, and at least 2 have gotten pairs for themselves.
'nuf said.

I am a frequent traveler and have run through several sets of headphones trying to find some that work well to reduce noise on airplanes. I've tried the Sony MDR-EX51LP Earbud Headphones, which were worthless for DVD viewing on my laptop. I then moved to the Sony MDR-NC11 Fontopia Noise Canceling Headphones earbuds, which were better, but not great (and they died in one year). I just bought Shure E3c earbuds and these Sennheiser PXC 250 Noise Canceling Headphones.

Of all these, the Sennheiser seemed to have the best noise reduction (though the Sony MDR-EX51 and Shure E3c are not active noise canceling, just "sound isolating") of the bunch. In addition to plane rides, I have started using them in long cab rides to help catch up on some Z's - the 15db noise reduction is very effective. If they weren't highly effective, I probably would not fuss with digging them out of the briefcase when not using in conjuction with laptop or MP3 player.

The Sennheiser are the only travel headphones I own that are not buds or in-ear style, and they seem to have very good sound quality (though I am admittedly not an audiophile). They are not really "open" cups, but they do not completely cover the ears like studio quality monitors. It does seem to be enough to make an adequate seal - helping further reduce outside noise and keeping fellow travelers from hearing your tunes. Since I generally use buds, I have found the PXC 250s slightly uncomfortable, but that may just be personal preference.

The Sennheisers fold up quite well, and are good for traveling (not as small as buds, but much smaller than the giant Bose headphones you often see on planes). The included travel pouch fits well and has an outside zip pocket for the airplane connector and a spare pair of AAA batteries necessary for the active noise canceling.

My biggest complaint is the same with pretty much all active noise canceling headphones - the in-line battery pack is cumbersome. Sennheiser did a good job of making the cord long enough that the pack can be easily hooked on a belt. However, you then have the wanna-be batman utility belt look that repels normal people :(

In short, if you want some great noise canceling headphones (and do not want to splurge on the $300 Bose headphones), the Sennheiser PXC 250 is a great option!