Hello. I see there is little info on the subject so I thought to chime in. Longtime reader, first time poster.
I have the M20d since feb 2013 and have used it at least every week sisnce then, including an outdoor tour of 2 months/37 locations in scorching heat, rain, and all the conditions a hot summer has. Also, this winter, every week outside, at temps as low as -10 degrees Celsius. The mixer performed beautifully, we are a band of 3, and the mixer also playes the backing tracks. Initially used it from stage, but after the first show and a little training the FoH guy took the iPad and mixed from the croud. He is a proffesional and trained to work with everything from a digital Yamaha 01R up to the Digico they use on large national shows.
Long story short, he liked the mixer, including the sound. It is a very potent mixer, with a basic interface for nonPros and a complex one, which has detailed clasaic parameters unveiled, foor those who know what it means. So if you need to "get more air" in the voice and have noo ideea how to, you use the basic interface, and if you want to apply 2.5dB of gain at 1Q at 6kHz you use the complex one. It has a few interesting features, as a dynamic eq, which proved interesting in use, and also i liked very much the integrated feedback killer. I am usually not fond of that type of devvice because it usually alters the voice. Weell, this one is monitoring and can use 12 notch filters that it apllies onthefly and we didn't notice a difference in the tone of voice. And the feedback is killed in no time, basically the lead could walk in front of the mains and sing from the public with no fear of feedback. Of course, it's not magical, but it is very effective. A usual chain of effects is like this on the voice: gain-eq-compressor-dynamic eq-deeser-sends of effects, monitor. I should also point out the gain, which is digital, and also automatic, if you set it. It means it monitors constantly the gain, and if you are overloading the input, it lowers the gain to a safer region.
I am very pleased with the purchase, and probably if needed I would buy the speakers as well, but we usually play in locations that already have more than enough. We only have 2 small monitors for me(keys) and the guitarist, 2 RCF 708A, realy small beasts, and an Sennhiser inear for lead vocal. We chose those because of weight, we loath to carry anything heavy, and they provide loads of power. Any other monitor present on stage we use as stagefill.
Hope that helps.