What's the best NAS for home usage with Macs?
Synology DiskStation
The Synology DSM software has built-in support for Macs including Apple Fileshare Protocol (afp) and Time Machine support. It has an iTunes server in addition to a regular DLNA Media server. As it is Linux based it is easy to configure the NAS to do rsync backups (ssh login).
It has a great web-based desktop-style interface that is very easy to understand and use. The DSM software is constantly being updated and improved.
They have solutions from 2-disk on up to 15-disk.  You can find the 4-disk DS411j without drives for less than $400.