I got my M2D2 last week and have had a chance to test it out. This was a replacement for a my first unit which presented tons of pink noise/distortion on chs. 1 and 2 after only 20 mins. My dealer was quick to resolve the problem and the new unit seems to present no such issues.
Some unorganized impressions follow. For reference, I do nature/field recording and run Schoeps Omnis in either AB or with a Jecklin disk. I have used the SD 702, the Nagra Seven and currently own the Sonosax R4+.
-Build quality: this bests every recorder/preamp I've ever hand in my hands. Body and encoders are Leica-quality. The screen is small but readable.
-The unit turns on/off in an instant. It's immediately recognizable as a Core Audio device on macOS Sierra and, to my surprise, works with the iPhone flawlessly with the basic lightening to usb camera kit that SAX recommends.
-Audio quality: The preamps/AD are on par with the R4+ which is the best sounding portable recorder I've ever heard. Headphone stage is equally impressive. I use the Sennheiser HD 25s, which are efficient, so I have no idea how it might handle more taxing headphones.
-Powering: I set it up to run 2chs with P48 ON and headphones to see how long the battery would last. I wasn't actually sending the 1s and 0s to my iPhone, but that said I got 5 hours before it powered down.
My expectations were high for this product, and they've been met and surpassed so far.
If it works for me in the field, I might sell the R4+. I only run 2chs, and I don't believe I'll miss out on sound quality with the M2D2.