It depends on the connection, do not expect a 100M/bits link.
I use a net5501 for my "all-in-one" box (file server (samba), printers
share, router, ...). The file server is not very speed but is enougth
for doing backups. (From time to time, backup the server to an external
usb disk).
I don't think a netbook will be reliable running 24/24.
This was my only concern on the net5501, the reliablity of the internal
2.5" disk drive, looks good after 3 years.
Check the soekris-tech mailing list, questions about performances are
often asked.