Personally, I've seen growing stability issues. Builds are failing for no apparent reason and work fine after a manual restart. Circe CI was the next free service to try.
Worth plugging https://sr.ht I guess. It's free for now and it's going to be very cheap[1] when it does start requiring a subscription. In addition to supporting the open-source developer who gave us things like Sway, you'd also be able to build on platforms like RISC-V, which I don't think anybody else offers.
FWIW, I'm currently using Circle for several builds, and am actually looking at alternatives now. The web interface has been falling over (which is highly disruptive to developers when they can't look at build results and trigger deploys, etc).
Circle has some nice features (especially easy artifacting and splitting rspec tests based on historic run-time) but it's not enough to overcome the lost momentum and productivity of being unable to reach the web interface.
At my company we've just finished migrating our Android builds from CircleCI to Buildkite (we did the iOS migration in December).
I've been very happy with Buildkite so far. The web interface fits my mental model of how things should work much better than CircleCI's ever did.
Our primary reason for switching was CircleCI's unpredictable and high costs on the iOS side (per minute billing combined with slow builds -> monthly fees in the price range of a new Mac mini).
Our Android builds are now about 10 times as fast at the same price.
The savings and the performance benefits are of course wholly due to the bring-your-own-build-machines approach that Buildkite takes. I don't see that as much of a hardship, though, especially since CircleCI 2.0 removed a lot of the "it just works" magic in favor of your own build scripts and Docker images. Preparing the build agents is not significantly more work.
