I've looked at OpenRC, RUnit and S6. I haven't recently run any of them "in production", however.
Personally, I am a strong believer that declaring the desired state is a lot easier to get right than actually writing the code to get there. Beyond that, I'm not saying any of these are bad at being what they are, systemd just has more features, some of which I really like. Two examples I'm actively using currently are automount units and socket activation (S6 also has socket activation). I have some remote folders mounted via SSHFS automatically when I access them and this is incredibly useful for my workflow.
Could I find tools to slot into other init systems that do this for me? Probably. But systemd has this neatly packaged up, easy to configure and easy to introspect state.
It uses a folder with a subfolder for every service. Each subfolder contains a script called run. The system runs the run script. If it exits, it waits two seconds and runs it again. Repeatedly. It's very worse–is–better.
There are commands to control the services and check their status. For example, if a file called down exists next to the run script, it won't run it. This is how you disable a service.
It checks for service folders being created and deleted. New folders are started, and deleted ones are stopped cleanly. They can also be symlinks, so you don't need to worry about deleting a running service folder and you can remove a service from init without erasing the scripts you wrote.
The whole system is useful in many situations and not only as pid 1.
Maybe one day I'll invent a runit–based distribution.
