Free Europe Tours: is a free tour in Prague worth the cost?

Angela leads a tour for FreePragueTours.eu. The company operates elsewhere.
Today, Sean and I took a nearly three hour tour of Prague. It was a free tour. But can you afford to take one yourself?
To begin the tour, our kindly guide - a beautiful young woman of German extract named Angela who has lived in Prague for 10 years - told us that it is custom tip, somewhere between 150 and 200 korunas. The money goes to her pay, promotion and distribution of the Free Prague Tours, she said.
Sean and I each tipped Angela 150 koruna, nearly $8.
These tours - free, aside from tip - are in Munich and Berlin, Germany and Amsterdam in the Netherlands - perhaps elsewhere too. It gives you the chance to walk around these cities with a (relative) local and (supposed) expert, for somewhat minimal cost. But I don’t know if this is always the best way to do things.

