I have to be the bad guy here and say that as a Pentax enthusiast and user, I think they lost it with the M series. Small and compact they were indeed, but IMHO only at the expense of ruggedness and reliability compared with the previous K series and Spotmatics. I had all three - ME, ME Super and MX, but the ME & Super are so different to the MX that I won't dwell on their shortcomings. I bought my MX nearly new, thinking of it as down-sized Spotmatic/KX, but although it wasn't very old the foam seals needed replacing and the slow shutter speeds were a constant source of trouble. It cost me a fortune in repairs before I gave up on it. Pity, because it was certainly nice to use and I really tried to like it. One other major gripe was the LEDs for the meter display which were not very visible in bright conditions (great, though, for poor light, where the Spot/K match needle disappeared into the gloom). Perhaps I just had a bad one.
Best wishes,
Steve