I've never used it. When I was considering it awhile back, the main problem was that out of the box it only supports SAMBA, not NFS and the only administrative access seemed to be via a Web based GUI (i.e. no access to the internals). But looking around some more, I found this page
which describes how to modify the firmware to add additional features. FWIW I eventually went with the Linksys NSLU2 with the uNSLUng firmware
and a 300 GB USB hard drive. It's served me well so far. Note that playing around with the hardware beyond anything described in the user manual or adding a custom firmware to any device like this will almost certainly void your warranty.