My friend A. did this tour on Monday, was quite enthusiastic and warmly recommended it.
Here's what she had to say:
"No need for booking, just pop up in Dam square at the start time (we made the tour in the afternoon).
Stephanie, our guide, was very enthusiastic and knowledgeable about Amsterdam, she told us plenty of stories and anecdotes!
perfect for cheese lover`s (not for me!), during the tour they even got cheese tasting.
and of course you pay what you want, Stephanie deserves a good tip :o)"
It's almost free, save for the 3 euros of the booking fee and covers, in a funny and entertaining way, the history of the city through its main sights.
The tours departs from the Dam twice a day.