For moderate levels and when I am in a relaxed state to begin with, I can leave the destructive interference feature off. The insulation all by itself is very effective. If the noise is really annoying (or if I'm anxious or tired or something) I can turn on the noise-cancellation. It can feel a little awkward at first and I have a vague sense that it might not be good for my ears, but essentially all external distractions just fade into the background. To interrupt me you basically have to walk up and tap on my desk.
I find it quite a disadvantage in my current office that when I need to do something detailed in a rigorous way I do not get to wear headphones.
I find this to be true also.
Additionally, I've noticed that during the planning stage music is less helpful/possibly distracting. However, once I've more or less decided what to do and am focused more on implementation music can help me focus.
[1] Sorry, can't find the source. Will update if I find it.
http://simplynoise.com/ has decent brown noise.