Wow! I bought these primarily to wear in a noisy open-plan office. Bose were too expensive so having ummed and arred I went for these. I looked at the more expensive PXC300 version but really couldn't see what the difference is, except the price of course. The noise cancellation is quite remarkable. Switch it on and all the background rumble of fans/photocopier/shredder/loud conversations and corridor meetings disappears. It is like sitting in a quiet room; concetration is once more possible. Then plug in to sound source (Sony MP3 and/or PC) and you can hear everything very clearly. Great bass response - though better on the PC than the MP3 player - I guess because the PC has more juice (MP3 is fine though).
The phones are small but fit snugly over the ears. The padding/quality is what you would expect from Sennheisser (excellent).
I was worried about interference but no problems so far. Incidentally the battery pack is about the size of a white board marker, or a tube of Rolos and not really the nuisance I thought it might be.
What a great piece of kit - the 'phones fold up (flat - the ear pieces swivel) and stow in the supplied zip pouch, which comes with jack adaptors (6mm to larger jack + double jack for some aircraft). Marvellous.