I've seen it in almost every shop in Hamburg. In other regions it's more a few group of people that drink it. In my university it's quite popular for instance.
It's now available in every larger supermarket even in mid-sized west-German cities, like here in Wuppertal. So somebody must drink it.
In hacker spaces it's everywhere.
The company I work for has always a few Club Mate in the refrigerator.
In Düsseldorf/Germany.
It's also available in Hamburg, Leipzig and other German biggish cities.
When I visited the university of Göttingen, it was everywhere. Students also seem to have it adopted as the drink of choice.
It's now available in supermarkets in rural NRW. So, I don't think "only Berlin" is remotely true anymore.