I used to run it. Initially as a VM on Debian, later inside a jail on FreeBSD. It worked nicely.

The main criticism I have of it is that it is a solution in search of a problem. Why would I use it in preference to either vanilla FreeBSD or vanilla Debian? I eventually made the move and just went to vanilla FreeBSD. It avoids the potential for any subtle incompatibilities you might encounter between the FreeBSD kernel and a foreign userland that was never intended to be used with it.

Don't get me wrong, it's a great technical achievement. But I'm sceptical that it has major value.