It's always tricky booking a free walking tour, some guides are ok or good, some are just aweful, ... and then sometimes you have a fantastic guide. Antek was one of those fantastic ones. We came to Warsaw without any knowledge, having booked very last minute and did not know what to expect. Even though Warsaw is not thé city of Poland to visit, Antek still managed to make the whole tour and his explanations very interesting. He gave the right amount of info in a precise way, so you could still follow the history of Warsaw. Absolutely recommend going on this tour!
Depending on the tour guide, you might have a good experience with this tour. For us, unfortunatly, it wasn't a good tour. We had a English speaking (native Spanish) guide, who didn't speak loudly enough, so a lot of the info got lost. Next to that, we had a too long explanation about Flamenco that kind of felt like a sales pitch to buy tickets of the specific venue to a flamenco show.
Ciarán is a great and very involved guide with a ton of knowledge and funny stories about Dublin and Ireland in general. Highly recommend going with his Company Unearthed Tours. Look out for the purple umbrella!