I have the same issue. I knew my M50 worked fine with my iPhone 4 so when it no longer worked consistently with my new iPhone 5 I went into the Apple Store where they exchanged my phone on the spot. Same issue. I contacted Plantronics and was sent a warranty replacement M50. Same issue still Sounds like an engineering issue to me.
By way of further info, my problem is that the unit has frequent dropouts when outdoors, to the point where it is simply not usable. Inside my apartment it seems to work fine, although the phone is never more than a few feet from the headset. Outside unless I hold the phone at chest level or above, I get very frequent dropouts.