I like my work colleagues, but sometimes I need to concentrate (or I'm just grumpy for some reason).
Or, you know, tell people to be quiet.
I completely agree with you, without music, noise cancelling headphones are pretty much useless to help you focus by canceling 'asymmetric' noise.
My personal setup is to have a pair of earplugs in resina as a first layer, my headphones as a second layer, the noise cancelling option on and finally low volume music. With that setup I can't hear anything to the point where I get scared shitless if someone disturbs me. That's how deep in the zone I become!
Solution for this was proposed in Diamond Age by Neal Stephenson. Engineers were working in big room, but they were so focused, guests had to use special protocol: stand beside desk and wait quietly until you are noticed.
I hate the modern corporate office environment. HR should just be upfront about what open plan offices are about: saving money. It lets you cram in as many people in as small an area as possible. That's it. Nevermind if they're productive or not. We're SAVING MONEY!
Which wouldn't be so bad except for the fact that company laptop is unusable for approximately 30 minutes after boot (or hibernate/suspend), when it tries to scan the whole drive for viruses or whatever.
I don't even want to count how many hours of productivity are wasted because of stupidities like these.
When headphones are effective at masking background noise, people sometimes tap me on the shoulder or knock on my desk. It can be really jarring if you're lost in thought and not aware of your surroundings. Interruptions are normally costly, but being surprised like that makes it so much harder to regain focus afterward.
I work in an open office of ~20 people and did exactly what you described - I brought my own keyboard to replace the standard issue flat Apple keyboard my company purchased in bulk. It has made a big difference in my typing speed. In fact there's four of us in here with mechanical keyboards and nobody seems to mind. But maybe that's because one of them is the CEO :)
We've honestly never had it come up as a problem.