The noise cancellation works; I can definitely tell that there is less low-frequency sound from the computer fans. Unfortunately there is a lot of mid-to-high frequency hiss when the noise cancellation is turned on. In terms of total perceived "noise", I'd have to rank, from best to worst:
1. Music playing on headphones, cancellation off
2. No headphones
3. Music playing on headphones, cancellation on
4. Wearing headphones (no music) with cancellation on
I need to give them a try in a noisier environment (i.e, the plane). But, the noise cancellation feature just isn't right for office use.