I am a maths and physics tutor and have been with this website about four years. I have found them easy to use and reliable. The fee they ask for, to exchange contact details, is very reasonable considering that it is a one-off charge per tutee. Agencies can ask 25% of the tutor's fee for *each tuition*!
Probably around 3/4 of the instances in which I have exchanged contact details have resulted in long-term tutees, lasting a year or sometimes two. This easily makes up for the few that back out before or shortly after the first session.