I have some Bose QC15s. Yes, they can be used without an audio input (indeed, you can disconnect the cable[1] and use them purely for noise cancellation).
As Edd says, the active noise cancellation is designed for continuous machine noise (engines, fans, that sort of thing), but is also astoundingly effective at "large room full of people" noise, and noticeable but underwhelming against continuous alarm beeping noises.
It does very little for clear human speech, loud music, impulse noise and so on, where most of the attenuation comes from the physical muffs of the headphones enclosing your ears. This is brilliant if you want to hear someone talking to you[2] in a noisy environment, where the active noise cancellation serves to improve the signal to noise ratio. It's largely useless when you want to hear yourself think over the sound of someone having a phone call next to you, or similar.
More generally, while the Bose active noise cancellation is second to none, they're only mediocre headphones. They lack the clarity of my late, lamented Sennheisers, the build seems a bit plasticy, and they don't fit my head[3] very well.
Top tip: Beware of misleading use of the term 'noise cancelling' to refer to either a) headphones which passively attenuate all external sounds and b) headsets with microphones designed to reject background noise.
[1] This model is wired only. Later versions allow for wireless input via The Devil's Other Radio.
[2] Though it comes with the serious gotcha that since you can't hear the noise the other person can't hear you over, you probably won't speak loudly enough to be heard.
[3] Most things designed to fit heads don't fit my head very well, so don't hold that against them.