I've been using Go Reminders for my massage business. Nothing is more annoying than remembering to send reminders to clients so they don't miss their appointments. Go Reminders is easy to use and has a clean feel and look. A couple things that I like are the appointments go into my Google calendar, I also like that you can edit your reminder message template. Lots of other services do not let you edit.
They always seem to be working on things to make your life easier.