Used an M1 for a year, quiet, reliable. Ran a 120 + 40 system in addition to a calcium reactor and a BRS GFO/carbon reactor.
A month ago I noticed that my Apex conductivity readings were ranging between 23 and 25 for no apparent reason. I eventually traced it to the M1. It has been replaced by a Fluval SP6.
(I needed a reliable conductivity reading as a way to fine tune my system's salinity, hence the need for the change)
I considered other DC pumps, but they all seem to be based on the same design so I stayed away for now.