I used Paris Greeters last year and had a brilliant tour.
Because of my total disorientation in the northern hemisphere, I was over 30 minutes late, but the Greeter waited for me (I had no way to contact him). We did a tour if the area around Opera, which used to include furniture and upholstery artisans (his family used to have a business there). I would neve have seen or known any of this without his input.
Because we started so late, the tour went through lunch time, and I needed to eat, so paid for his lunch. The Greeters are a no tipping group, bit I made, in advance, a donation.