When I had my store, I would regularly get Miele's in that were 10-15 years old for service. The most problems I saw residentially were that the machine was simply clogged with a sock, clothing tag, etc. that simply needed to be removed. I think I saw a broken tool lid, but only once. Only $20 later, the machine was like new.
I would also get machines in that were used by Molly Maids, a cleaning franchise. These Mieles had been used to clean six houses per day, five days a week. These girls are TOUGH on vacuums, believe me. One machine in particular came in that was fifteen years old. After fifteen years of all that use, the wheels had fallen off, the tool lid was broken, and the hose end needed to be replaced. But the actual plastic body of the machine was fine.
The only problem that I can ever say Miele encountered on a wide basis due to poor plastic was a cord reel problem that they encountered on the 300-500 series machines. Two little plastic tabs broke off on the center post of the cord reel which caused the reel to come away from the electrical contacts and subsequently the machine would just shut off. Unfortunately several people encountered this with their machines, but Miele did beef up the cordwinders and there hasn't been a problem since.
In a nutshell, the plastic that is used on Miele's is great. Not indestructable (like anything else) but the fact that I and several other vac shops were getting in 10-15 year old machines that only needed minor repairs spoke volumes. It should put to bed this whole "plastic is bad, metal is good" myth.
