I like how easy it is to use, as well has its ability of integration with other platforms such us Zoom or Slack! It's not free but if you are looking only in this aspect then don't worry because there're many alternatives out here that can do what circleback does (and more!).
One thing i dislike about circles back was when they had an update last year where their logo changed from white background into black one which made me feel uncomfortable at first glance until now everything works fine again so no issues anymore :). If your team uses any kind communication platform be sure before buying anything else just try them all see whats best fit for ur needs/business requirements :) We used Circles Back mainly during our sales meetings using zoom chat feature we also integrated slack sometimes too!