Updating ZFS-based FreeBSD installation to 8.1-RC2 was rather easy, but time consuming: it took more then 2 hours.
1) freebsd-update couldn't fetch updates from first try. freebsd-update is ugly thing. Comparing checksums file-by-file, sorting lists of files on disk and other "lovely" habits... Source update would be faster.
2) this good tool erased my zfs booter, had to boot from 8.0 dvd and recover it.
3) However, i386 FreeBSD 8.0 runs on ZFS quite stable (768 Mb of memory, no any tuning - 160 Mb arc, kmem_size_max - 330 Mb, kmem_size - 256 Mb (defaults))