CircleCI

Unless you have a reasonably large account, you're going to be mainly stuck reading their documentation. Which has improved somewhat over the years but is still extremely limited compared to a platform like Digital Ocean who invested in the documentation and a community to ensure it's kept up to date. If you can't find your answer there, you can be stuck.

I haven't personally used their support service, but I have heard from others that they are responsive. I've also seen only one or two downtimes in over a year of use and both were no more than an hour or two.

The support team of Circle CI is good enough, we've some situations when the support team helped us very well and fast. But in some other occasions we've to wait a little amount of time to get a successful response to our problems.
I think that the support service of Circle is good.

I would rate the support for CircleCI highly. They're a small team but they've got some of the most dynamic, global teams available to help.

I have not been in touch with customer support, and as such am not able to rate this aspect of CircleCI.

CircleCI support is pretty good but the company is small and a startup. It's not got the type of enterprise support you would see from larger institutional companies.