Before Circle, I led multiple engineering teams, building multiple products from "nothing" to "shipping, and making lots of money". I set up buildbot, hudson, bamboo, jenkins, and some other things I don't even remember the name of. I was always the build guy, I was always the one that said "yes, we will be testing, yes, the build will be green." Those were controversial statements back then.
I've been there, done that, and I didn't want to set up Bamboo again. I didn't want to debug why the tests failed. That's why I built Circle.
Our customers, most of whom switched from Jenkins, will tell you we are a steal.
Maybe your project is still simple. Maybe your platform is fairly stable. But for most professional developers, keeping CI running and green is a challenge. Even at Circle, I spend more time than I'd like debugging the test suite. But we're fixing that. Every day is spent making devs lives easier.
Jenkins costs you nothing per month, but your developers cost money, and Circle's hardware is faster than digital ocean, and the cost of keeping the build green on Circle is lower than the cost of keeping it green on Jenkins anywhere else. If you're running a startup, or a tech company, odds are very good that salary is the number one cost center. Optimizing your Digital Ocean bill is penny wise, pound foolish.