As a counter point, we had major issues with just paying for it via CircleCI. Excessive downtimes, a UI so slow it was nearly unusable, etc. We decided at the time we couldn't bet our company on it and we moved to Jenkins. We have far more control, build time and build costs are lower, uptime is better (All of this at the expense of initial dev time and maintenance of course). Generally we are a SaaS company, but paying for a solution just doesn't always work when you get larger and when there are limited options out there.