I don't use FreeBSD .. yet ...
but I do love ZFS..
OpenBSD will never have it due to the NDA
or something like that ...
Theo is very adamant about NDA's and the like.. (no holy war intended)
On the other hand.. FreeBSD is more flexible.. AFAIK...
ZFS root is workable on FreeBSD per this site..
http://www.ish.com.au/solutions/articles/freebsdzfs
hth
rk