10 years on and off only with MM. Part of the reason I stick with Minimed is because of the reliability of their devices. I only had one serious issue with an occlusion years ago where the occlusion alarm didn't sound.
The FDA tracks self-reported incidents of device malfunctions. In general, the issues associated with insulin pumps is very, very low. Yes, malfunctions do happen. It is a device and it can break or be damaged. There's also the element of user error.
The best way to mitigate the risks associated with a potential malfunction is to test, test, test. That is one reason why the device manufacturers are so insistent on you consistently testing. This way, you're apt to catch any issues before they become serious.
I think non-delivery issues are the most common with pumps. Situations in which the pumps deliver too much insulin are rare. To be safe, I keep my pump away from any x-ray machines, as there is the possibility that they could damage the pump and cause it to malfunction.