M1 is easy to temporarily put on or off the guitar when needed. The description says that the pickup fits soundholes that are 90mm or wider, which is not really true. I could not fit the pickup on Martin 000-18, whose soundhole is a bit more than 9,5cm.
One other disadvantage is that the input place for the cable is located at the side of a pickup that is not really accessible without loosening the strings. Not particularly practical for those who do not want to permanently install the pickup.
M1 is a pretty sensitive magnetic pickup but not in the sense of being prone to feedback. It is sensitive in a way that it picks up a bit more than just strings next to it. It will pick up occasional noises caused by plectrum and if you hit the pickup accidentally while playing, you will hear that as well. For some players this might be a deal but for some others, having a sensitive pickup might be a plus.
M1 has six pins that can be individually adjusted to fit many guitars and it does deliver certain acoustical characteristics of an instrument and it sounds less “electric” than many other magnetic pickups. Since it is a passive pickup, it requires a good preamplifier in order to get a good and clean signal. I have tried M1 on a couple of guitars and I must state that I did not get good results with every guitar. In combination with certain guitars, M1 delivered unpleasant resonances that made it unusable so, at the end, I kept it on one of my guitars and took another brand for other guitars.
