I was in Berlin yesterday, and had no problem getting a taxi to the airport (THF in my case). We did book it the day before (knowing there was going to be a public transport strike) but saw plenty of empty taxis go past our hotel while we were waiting for our prebooked one. Things were remarkably normal in Berlin yesterday - we expected to see lots more cars on the road but there weren't; everybody must have either used the S-Bahn or cycled to work.

Of course if the S-Bahn goes on strike as well (it's on the cards) things could be different.