Another happy s6 user here, and pretty much for the same reasons. I like the clear supervision process tree it creates. I'm not sold on the execline syntax, but it's relatively clean and easy to work with and since I already use a configuration management system, generating custom startup files (from a single template) isn't that much of a challenge.

I'm currently exploring how to generate minimal service environments from Guix package manifests. Being able to spawn svscan as a child under the root supervision tree means I can create each environment with its own service directory, and never worry about cross-contamination from whatever else is on the host.