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.
