As an end user I've been pretty happy with it. I've used Jenkins, Travis, Github and Circle mostly, and of those I hope to never have to use Travis or Jenkins again, and I've only really used Github for some simple stuff, but rather enjoyed the experience.

Circle however I've used with some fairly lengthy and complex build processes involving multiple parallel builds of Rails apps during upgrades and it's handled the workflow well.

I think they've improved the UI a bit recently, but it's still pretty complex though.