Pros: Good Sound, Comfortable, Great Battery Life, Talk-Through Button, Passive Listening Mode
Cons: Odd Button Placement, Not Real Portable
My experience with noise-canceling headphones include the Bose QC-15 and the Parrot Ziks.
In comparison, the Sennheiser PXC 450s sound really nice. The sound is clear and revealing. Good bass, but not muddy. They will expose poor quality MP3s. They are very comfortable and can be worn for long periods of time without any discomfort. I think the Bose are slightly more comfortable, but not by much. They are a large headphone set, but they fold up for travel, and they come in a zipper case with a couple of adapters. They seem to be well built. The noise-canceling is not quite as good as the QC-15s, but the sound is much better. A fair trade. They are not noise elimination headphones. You will still hear (faintly) the sound of crying children on the plane. The battery (AAA) life is very good and they can be used once the battery is dead. A great feature. There is a slight loss of bass when using the headphones sans noise-canceling. Also, there is a talk-through button that mutes the music and uses the microphones to here the outside world. This is handy at counters and when someone is asking questions, but be ready for odd looks.
Some negatives: The talk-through button does not pause the music when used. Also, the volume controls on the headphones change the volume on the headphones, not the iPod. The cord is does not have iPhone controls.
Since buying them, I think they will replace my Bose as my choice for travel. I use the Ziks mostly for walks around the neighborhood because of the wireless function. Good combination of noise reduction and quality sound. Recommended.