I found that the Bose noise-cancelling headphones are great at substantially depressing the perception of constant noise--like airplane hum, fans, and a crowd of voices. They are not, however, at least to my ears, at all effective in muting individual voices or any other sporadic sounds. So if you work in a boiler room atmosphere with lots of people all talking at once at similar loudness levels, I think you'll be happy with them. If, on the other hand, you work near a few people who talk on the phone, or to visitors from time to time, then I don't think noise-cancelling headphones are what you're looking for. I guess you understand the physics behind these things (or can easily find explanations), so they are intrinsically limited by their enabling technology.
For myself, I found that other than on airplanes I prefer to use noise-isolating in-ear phones. The ones I have use a foam eartip which you compress between your thumb and finger, insert into the ear, and then allow to expand to fill the space and mechanically block the sound. The sound is transmitted by a tiny tube that runs through the center of the foam. I've found these to be highly effective at blocking sporadic or irregular noises, and especially voices. I think there is a wide selection of similar products available, but the ones that I have offer great sound quality as well as isolation, but again at a not insignificant price. I use the Etymotic ER-4P, with the foam, not the soft plastic, eartips; both are provided in the package.
http://www.amazon.com/Etymotic-ER-4P-Portable-In-Ear-Earphones/dp/B0007WZLDC
They also have the advantage of easy portability (my Bose headset is pretty bulky) and need no batteries.
These are supposed to provide 42dB of attenuation, so if there's a phone you need to hear, you might need to buy a flashing light.
