I'm no expert, but ZFS support on FreeBSD seems to be pretty solid (more or less on par with Solaris). FreeNAS seems to be quite popular and it's based on FreeBSD and ZFS.
If you have a question about a specific ZFS feature, drop by the FreeBSD forums (http://forums.freebsd.org) and ask; the FreeBSD community is generally quite friendly and I'm sure you'll get a good answer to your question.
For general information about FreeBSD + ZFS, check out these links:
http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/fi...