We used it a lot last year to cover the shortcomings in maths teaching that became apparent as the terms progressed, and became pretty close to two of the tutors we used. Apparently, we were early adopters so I was asked to give feedback particularly. We thought it was very good, and much more convenient than face to face, plus there's no geographical constraint.
Take time to choose the tutors by reading their biographies and considering who the child will click with best. I short-listed those that sounded promising, based on subjects first, obviously but also the tutors' other outside interests, because naturally there needs to be a little small talk. DS did most of the pre-selection and then I sat in on the last few minutes of the free 15-minute 'interviews' but I left him to decide who he preferred.
All the sessions are recorded so you can play them back, and the help desk was good when we had an issue with broadband speed (yours needs to be adequate/quickish).
We never found a tutor for AS Physics that we could like, but that might be because we didn't keep looking for long enough.
One of ours also quit the site (after nine months) because he was tutoring face to face and earning a lot more doing that but My Tutor were not prepared to alter the split of fees in his favour as much as he wanted.
On balance, it was generally worthwhile.