The previous post more than adequately answer the question what one can do with a PowerPC Mac today.
Regarding your query is it worth buying, that depends on your needs, the price being asked, it's condition and if it's fully functional.
If in excellent condition and fully functional, depending where you're located, anything up to 200euros (US$225) imho is worth considering - especially for the 17" 1,25GHz model which I reckon is the best of G4 iMac's and becoming more difficult to find. I have a number of different models including the 20" - all of which I've completely refurbished, but using the 17" and 20" regularly I still prefer the former. I stress that the 1,25GHz is the one to aim for and not the 800MHz, or 1GHz (USB1) models.
Pls let us know if you go ahead with the purchase.