I have an M500 that I've had about 5 years now. About 3 years in it quits working, talked with the lady at Maytronics , did a little diagnostic over the phone and ended up sending it in for testing. Seems the motor got goofy and only a few days after my 3 year warrantee ran out.. They honored my warrantee anyway, they replaced the entire power unit and sent it back! Skip ahead a year, lightning struck my house and killed it again (along with my hot tub, a couple computers, and a bunch of other stuff) Anyway I sent it back in they replaced the power unit again at a nominal price and I had it back in a week.
In my opinion customer service there is great!! Would not hesitate to buy another! My only thing is I bought the M500 over the M400 because it has a remote, I can drive it o a dirty spot or whatever.. I NEVER USE IT, wish I had saved the extra $100!