I've getting excellent performance with a 4-disk SATA 2 ZFS-R1 setup...it'll max out my 1GB network, though it'll occasionally pause to flush out the cache with larger writes. Part of that could be the other end too...but performance is better with this than with Windows Home Server 2011 and the disks on a Smart Array P410/512 controller.
The only reason it's not on the controller is the lack of SMART and spindown support...the hopes is that I can have this box up 24/7, instead of shutdown when idle (how I had WHS2011 setup), mixed with WOL...