I first ran into Acuity as an end-user, scheduling a massage. I was so impressed by the simple, attractive design that I decided to try the tool for my own uses (scheduling software usability sessions). It has dramatically improved my workflow and makes it so much easier for participants to find a time that works for them, without lengthy back-and-forth communications.
As a UX expert, I want to mention that Acuity's admin tools are very easy to use and the feature set is comprehensive. The free version has been great and I'm now looking for budget to upgrade my plan!