We at Pixelstorm use Calendly for our internal systems and for clients we proposed Acuity or Calendly.
Calendly is easy to setup and low cost. It's such a user friendly interface, works really well with Google Calendar and other calendar systems. The price point is low and allows for many different calendars. Customisation is easy as each calendar can have certain time lengths (ie: 15 mins or 30 minutes) and block out windows of time.
A feature we love is the auto time zone changer. So if you are working with clients on the other side of the globe
Cons are the sophistication when compared to Acuity for things like ecommerce and other clients. For us, it's perfect