It depends how much they're selling it for, and what you plan to use it for, specifically.
I got my G4 a bit before the Mac mini was announced, and it is much much slower than the mini (350mhz G4), but I have a good chunk of RAM, and I am able to record 8 tracks simultaneously though my Delta 1010LT. I have some MIDI problems with the Delta that M-Audio says is a result of my 'slow' processor, but I think otherwise (since G4 processors are much more powerful than their clock speed indicates.)
You can always get a processor upgrade, though some companies do not support them officially, while it's most likely you will have no problems with them.
If it's going for under $500 then I'd say it's a good enough deal. Also be sure it has more than 128MB RAM.
(Even though Mac mini's cost $500, you won't get PCI slots, which was a must for me since I use a Delta). If it's over $500, I'd say get a Mac mini and some kind of Firewire or USB interface.