etcd has had significant improvements in lifecycle management and stability in the last year. You can learn a bit more about the upcoming etcd v3.0 release here: https://speakerdeck.com/philips/etcd-mission-critical-key-va...

For the most part we are recommending folks use Kubernetes for new projects over fleet. Kubernetes "completed our sentence" a few months after we started fleet and has rapidly matured. The kubernetes v1.3 release bring a number of new features as well: https://coreos.com/blog/kubernetes-v1.3-preview.html

And on your btrfs question we moved off of btrfs onto ext4 + overlayfs as soon as overlayfs became a viable option in late 2014. You can read more about that here: http://thread.gmane.org/gmane.linux.coreos.devel/1952

As for users and maturity some of the videos from Tectonic Summit Day #1 from this blog post might be useful: https://tectonic.com/blog/tectonic-summit-wrap-up.html

Happy to answer any other questions on IRC or email: https://coreos.com/community