I have used the same pair daily for two years. The technology is genuinely effective and it is effective at a narrower range of things than the marketing suggests.
What it actually cancels
The physics determines the answer and it is worth understanding.
Active cancellation works by generating a sound wave that is the inverse of the incoming one. For this to work, the system must predict the incoming wave accurately.
Low frequency sound has long wavelengths and is generally continuous and repetitive, which makes it highly predictable. Engine noise, air conditioning, road rumble, aircraft cabin noise.
These are cancelled remarkably well. The effect on an aircraft is dramatic and is the single strongest case for the technology.
High frequency sound has short wavelengths and is generally transient and unpredictable. Speech, clattering, notifications, a door closing.
These are cancelled poorly, and what reduces them is the physical seal of the ear cup rather than the electronics.
The open office problem
Which follows directly and is the most common disappointment.
The thing people most want to block in an office is conversation, and conversation is the thing active cancellation handles worst.
What works better in that environment is passive isolation — a good seal — plus something to mask the remaining speech, which is why people play music or noise rather than relying on cancellation alone.
Well-fitted in-ear designs frequently outperform larger over-ear models for speech specifically, because the seal is better, which is the opposite of what the specifications suggest.
Fit is most of the performance
The finding that surprised me most.
For in-ear designs, the seal from the tip determines the isolation, and the supplied tips fit a minority of ears well.
Changing to a different size or material improved my isolation more than any difference between models I have tried.
For over-ear designs, anything breaking the seal degrades it substantially, including spectacle arms, which is a real effect and rarely mentioned.
Which means comparing models on specification is close to meaningless, since the dominant variable is how a particular design fits a particular head.
The things that got worse over two years
Ear pads, which compress and harden, degrading both comfort and the seal. Replaceable on better models and not on others, which is worth checking before buying.
Battery, which follows the usual curve. Runtime is now noticeably shorter and still adequate.
The hinge, which is the mechanical failure point on almost every folding design and which I have learned to treat carefully.
And the application, which has been updated several times, has moved settings around, and now asks for an account, which it did not originally.
The health question, briefly
Worth addressing since it comes up.
Cancellation itself does not damage hearing; it is the absence of sound rather than additional sound.
The genuine benefit is that it allows listening at lower volumes, because you are not competing with background noise. Listening loudly to overcome noise is the actual risk, and cancellation reduces the need to.
Some people report pressure sensations or mild discomfort with strong cancellation, which is a known and individual effect, and most devices allow the strength to be reduced.
Awareness is the other consideration. Wearing these while walking near traffic or cycling removes information you need, and transparency modes are a partial answer rather than a full one.
What I would buy again
Something with replaceable ear pads, which extends the life considerably.
Something that works passively with the electronics off and the battery flat, since some designs are unusable without power.
Something with a wired option, for the same reason and for latency on video.
A physical control for the transparency mode, because reaching for a phone to hear an announcement defeats the purpose.
And I would spend the effort on tip or pad fit rather than on the next model up, because that is where the actual performance is.
Flying specifically
Since it is the strongest use case and has its own practicalities.
Cabin noise is exactly the continuous low-frequency sound this technology handles best, and the reduction in fatigue over a long flight is the clearest benefit I have experienced.
Worth checking battery life against the flight duration, since running out halfway is a poor outcome and some models are unusable without power.
A wired connection is useful for aircraft entertainment systems, and an adapter is occasionally needed on older aircraft.
And cancellation can produce mild pressure discomfort during ascent and descent, which some people find noticeable and which is resolved by turning it off for those periods.
Repair and replacement parts
The practical longevity question.
Ear pads are the first thing to fail and are replaceable on some models and not others, at a cost that is trivial relative to the device.
Batteries are replaceable on very few models, which effectively sets the lifespan for most of them at the point the cell degrades.
Headband padding and hinges are common failures and are rarely serviceable.
Checking parts availability before buying is a reasonable filter, and it eliminates a surprising number of otherwise well-reviewed models.