I bought one of these back in the day, but even back then, brand new as it was, I thought it was a slow and limited computer.
It has some merit as a compact retro software machine, as it will run both Classic OS 9 applications, and OS X PPC applications. But I don't see how it is viable for any modern use--maybe you could use it as a slow file server or something.
Probably the oldest Mini I would consider is the 2006 Core Duo. At least it has two cores, gigabit Ethernet, and the Intel processor can also run Windows or Linux. Plop in an SSD and it might even feel peppy.