Most of my experience with Mac plastics have been good ones and the only problem I ever had was with a 5200, whose plastics snapped as soon as you put any kind of pressure or strain on them. That 5200 came from London originally and it led me to believe that the brittle plastics were possibly caused by the warmer, more humid climate down there. Possibly just coincidence though, but all of my other desktop Macs which have always lived in Scotland don't have any plastics issues.

Having said that, a small piece of the plastic "foot" under the front of my iMac G3 has shattered - it seems a rather flimsy piece of plastic to put the weight of the CRT glass on. Fortunately it is still usable.

Sorry to hear of your bad luck with your iMac. Fortunately G3 iMacs are available very cheaply these days so it should be easy enough to get replacement plastics. Annoying though!