True, FreeNAS could be faster, although I get about 7-8 MBs transfer rate out of it both ways (on a 100 mbit network), although I'm usually on WIFI, which is about 2 MBs at max, so who cares? Good enough to play music on a few PCs simultaneously, plus a divx here or there...
Although 0.7 nighly builds are coming out now, I think settting up user authentication isn't too complicated, there are good info in the Knowledge Base or on Sourceforge. And no need to know much about BSD either, just enable SSH, start WinSCP, and that's it: create folders, set permissions in properties, good 'old' fashioned Windoze-way...