I run ZFS with Raid Z on (5x) 2TB drives, the implementation works very well, and i like the idea of being able to increase the storage pool as I go on. I do know though it is fairly resource intensive (with FreeNAS) and is recommended ~ 1GB memory per TB (I currently have 8GB which runs the system quite well, not bottlenecks). I'm not really sure if the requirements would be different for Windows Server 2012.