The NFS addin is great.  I've been throwing VMWare ESXi 4.0 at the DNS-323 and have had a great experience so far.  I have nothing else that's remotely close that can stress my NAS like this.   
At the moment I'm writing a sustainable 5.9MB/sec (47Mb/sec) and reading 17.9MB/sec (143 Mb/sec).    NFSD is at 33% while writing and 83% while reading.  It seems the io subsystem is the bottleneck for performance.
What's great is with all of the random reads & writes expected from VM/ESXi, NFS doesn't seem to be unstable whatsoever during operation.  
My only gripe is when I reboot the 323, I lose my NFS config.