I'd say yes, worth buying and trying.
I still use a G4 but only for text related work;
the hardware is fine, no problems there.
So, it comes down to what you do in Photoshop.
1.something Ghz is plenty fast enough to do lots of routine,
low-intensity work in Photoshop.
If it's got the right monitor connector for a monitor you already own,
and you've got the software, you can't go wrong for $30.
At worst, it won't do what you want and you'll have to pay another $30 to have it recycled.
Not much of a downside, pretty good deal in my opinion.