Very common, but no one says 'one', it is archaic and posh. It's not my cup of tea, is it your cup of tea, they are common :)

As you might've read above, it's widely understood in the US, if not widely used. And it's about demographics: Older people use it frequently. Among younger people, it tends to indicate a certain personality—a little offbeat or nerdy, or maybe just generally reserved. I also suspect it's more common in rural areas, because I've grown up hearing this phrase frequently, and I'm in the Midwest. Extra note: I think most younger people (under 40 years old or so) who use the phrase do so specifically to indicate a slight revulsion to the thing while still being polite. If a certain food makes them nauseous, for example, they might say it's not their cup of tea, to sound polite.

I live in the United States. I use this expression myself. Everyone seems to understand it. Nobody has ever commented on it. So, despite the fact that coffee is a far more popular drink, the expression persists as an idiom anyway.

It's also common in South Africa. I think it may be one of the expressions I hear most often.

Not in the United States.