Home for start-up business? Yes. For tech start-ups? I don't think so. I have yet to meet one engineer-driven tech start-up from Berlin. Most founders I met and know are some business guys with oh-so-clever ideas looking for just the right CTO guy to make their plan reality. And most German developers I know also never thought about working for a start-up. They happily flock to every large German company you may know…
Edit: they could be an outlier, of course.