I have never lost it in flight, but I have had it refuse to work at start up, both in the plane and the car. Happened so often that I programmed the XM web address into my iPad, so that if XM didn't boot up I could call for a re-activation signal, as long as I was within cell phone coverage. This usually worked, but once it failed when I was in the boonies with no cell phone service, and isolated thunderstorms.
I have ditched xm wx and gone to ADSB-in. Save $400/yr too.