The biggest problems with the G3 iMacs are the PAV board and the internal plastics. Both are punished by the heat and tend to fail eventually in the case of the former, or become brittle in the case of the later (especially the slot loads). Some enterprising individuals might take the time to replace or repair the PAVs, but I would imagine G3 iMacs will become increasingly rare over time.

On the plus side, the logic boards are decent. I rarely hear of failure on that end, and G3s usually run pretty cool.