I've been watching the progress of my favorite filesystem, ZFS, in FreeBSD for a while now. ZFS v28 has been under test by the members of the freebsd-fs mailing list, and I have read multiple resources that have indicated that the performance of the ZFS implementation in FreeBSD is much better than in OpenSolaris; indeed, my Nexenta box only supports up to ZFS v26.
And now, ZFS v28 is now in FreeBSD-CURRENT:
There is an excellent article in BSD magazine with some general information. The article is by Martin Matuška, who maintains a ZFS v28 patchset against FreeBSD-STABLE.
When I have some time, I'll be excited to migrate to FreeBSD, and to get away from the insanity that is (was) OpenSolaris.