We used it for DD this year who was 18 months at the time. She didn't do much since we put our DD to bed shortly after she came (we had adjoining rooms so the sitter was able to sit in the other room and read, watch tv, whatever), but she was absolutely wonderful with my DD when she was up and she put me completely at ease -- and we had never left our DD before with anybody except her nanny and grandparents.
It is, however, expensive, and when our kids get older I think I would use one of the Kid Clubs or Cruisies instead. For now, however, it is a great service.