I recently visited Berlin with my girlfriend and we took the free walking tour. Seb was our guide. In short, it was absolutely wonderful!
The tour was pretty much the same concept as other Sandemans tours that I've been on around Europe: the standard tour is given for free, but the guides work for tips. I've always ended up giving about 10 Euro, but you're not under any obligation and the guides don't tend to be pushy (Seb certainly wasn't).
So far I've done the free tours with Sandemans in Amsterdam, Paris, London, Copenhagen and they've all been good; however, the Berlin one was the very best I've had so far. We'd arrived in the city a day before and, I have to admit, that I wasn't sold on the place. However, Seb was knowledgeable, funny and really brought the city to life for me. I'd liked to think that I was pretty well up on the history of the city before arriving, but I ended up learning a great deal, for example, about the specific events that led up to Hitler's rise to power and later the miscommunications that led to the fall of the Berlin wall.
If we had more time we would have probably done one of the company's more specific paid tours or their organised pub crawl; however, our itinerary was already too full.
Nevertheless, I fully recommend the free tour. I think most new visitors to the city would enjoy it and get something out of it.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.