We did a free tour and an twilight tour so I'll kind of split the review in two.
Free tour: our guide was Ben. He was nice and gave us fun and quirky info about Berlin becoming what it is and Berlin life today. We've seen some great street art, great places and heard many interesting stories. This is a great way to get to know the other side of Berlin (for many of us this is it's main side :)) and I'd recommend this tour to all people coming to Berlin for the first time or getting to know this side of Berlin for the first time but I don't think it would be thrilling enough to people much into street art or knowing alternative Berlin.
Twilight tour: our guide was Liam and he was great - very friendly, easy going and relaxed. He had may stories to tell, many interesting things to show ( among other things: vietnamese "shopping center", abandoned train wagon in the middle of some bushes, roof of an abandoned factory, bars...). The group was small - only 10 of us which made it very relaxed. We liked the tour very much and had a great time.
I also recommended them to friends and I'm planning to do some other tours next time I'm in Berlin :).More