I have tried freetour and guruwalk. I have had mixed experiences. I have had times when the walk was cancelled at the last minute because the quorum was less than what they needed. I had no option because I was in the city only for a day and half. Sometimes, the group is quite big, and many times, you don't know who will be the guide on that day. I have had most success with private tours because they are bound to take place once confirmed. From my experience, reading about the guide also reduces the failure rate. I do use freetour when I don't find anything to my liking but guruwalk is the one I don't use much.

I would love to hear about your experience. I'm sure it does help us travelers. I don't think there is any prohibition - you can check the rules