Noise canceling headphones?

I have been reading about the noise canceling headphones from Sennheiser and some other brands. Can anyone that has tried these give some insight in to the validity of this technology?

I flew United Business Class the other day, they gave you noise canceling headphones. When I turned on the noise canceling, it was like pushing the loudness button on an old amp. Not very pleasant. I would like to know whether the Sennheiser et all are any better?
I have not tried them, but since I'm flying to Italy in two weeks I started doing research about the problem of noise on airplanes. At they don't recommend noise canceling headphones even though they sell them, they recommend Etymotic Research ER-4 earphones, since they provide 23db of isolation as well as the flatest feq response of any headphone!! I just bought a pair (arrived in two days). They sound great. They take a little getting used to and are expensive ($269 from Headroom, $330 list).
I second Mg123. I have tried Sennheiser and one other (can't remember which one it was) noise cancelling. They do work--any monotonous noise is cancelled to some degree (like plane engines), but their fidelity is not very good. I bought a pair of Etymotic Research ER-4S (I recommend the S version) about 3 years ago. I have also purchased the airhead amplifier from headroom. This is a GREAT combination for travel. The ERs don't cancel noise, but they plug into your ear and effectively reduce the noise (I've heard by 25db--but perhaps its 23db--anyway it's a lot). They are fabulous fidelity. The airhead amplifier takes the whole thing up another level. Headrooms circuitry allows the user to switch in a little bit of cross talk with delay. This makes the sound less "in your head"--it is far less fatiguing than listening to regular headphones. Once you use one the headroom amps it's hard to listen for any length of time without it. For any audiophile who travels by plane very much--these are two products I would highly recommend.
I have a pair of the Sennheisers and have mixed feelings about them. Even with no noise around you, switching them on severely alters the timber of the music. They do cancel out some noise and boost the high frequencies, but they are not that comfortable and turning on the canceling feature often makes my ears pop (like changing elevation). I've use them on several trips and wished they canceled even more noise than they do ( only about 10db, and none at higher frequencies). Bottom line, they do help, but if I had to do it over, I would go to the Etymotic.
I have tried a cheap Brookstone and the Sennheiser NC headphones and agree with what's posted above. I do find it relaxing on long flights but the sound quality is not very good compared to the better Sennheiser's and Grado's. I have not tried the Etymotic.
I fly regularly between NY and London and I have both the etymotics and a pair of Sony noise canceling headphones. I find the Etymotics a bit thin sounding and quite uncomfortable after an hour or so, although the custom ear pieces might help. Has anyone tried custom ear things for the etymotics? The Sony phones were also pricey ($300?) and they don't sound great but I find the noise canceling circuitry very effective and a great relief. I should point out that they are soft, rubber, in the ear type headphones and not over the ear which perhaps makes a difference. I feel less fatigue from the flights and enjoy the in flight entertainment, DVDs, MP-3 files etc. In any case the guys at Headroom are great and I love their amps. Good luck.