Just a continuation to the older expired thread: This tour was superb! It would be literally 'free' if you had the gall to walk away without tipping anything, but in the words of the guide 'you just pay what you think it's worth' - they work for tips as they will say but once at the beginning & leave it at that (no sob/hard luck stories). Our guide was a graduate of German/History (also Russian) so extremely knowledgeable, passionate & engaging at what he does.
Did the 'Free Tour' in which I probably learned (or absorbed!) more in 3.5 hours than I did in the same amount of years at school, covering the 'majors'; Brandenburg Gate, Checkpoint Charlie, etc. We then did the Sachsenhausen tour the next day, since it was going to be with the very same guide - not something I ever saw myself doing, touring a concentration camp, but I'm glad I did it; very profound & moving.
So, I would recommend them highly. I also recommend that people really do hand over what they think it's worth in real terms, rather than 2/ 3 euros 'cos it's supposed to be free anyway'; from my point of view, I don't think I would have done much better on a *paid* tour.