As a 500 series and ex-DSP 5000 owner, I have always been pleased with M's service approach. I've had some e-mails left ignored but I've also had some answered out of hours. Calling M in the UK has been a good experience more often than not. I had to return a 598 twice for the same problem although it was finally fixed and returned free of charge. So, overall, a mixed bag but satisfactory on balance.
If I were an 800 series owner, though, I'd be upset at that kind of service. To 500 series owners who buy second hand and are delighted with whatever M does for them, I'd say "get some skin in the game" before apologising for M. Service is extremely important at that end of the market and, for me, a mixed bag of consumer experience, as in this thread, isn't good enough.
I wonder if M should consider a premium service for 800 series owners or even making a charge for repairs in return for greater "precision" in managing returns?