Pros
M3 can offer an ever-changing variety of opportunities on projects, which can include visits to the construction sites themselves to see the tangible results of years of effort. This is not for everyone, but for those who enjoy challenges (technical and otherwise) and who enjoy getting in the field M3 can be a pretty cool place to work. There are a lot of great people at M3 that are willing to answer questions and provide guidance to new or inexperienced people, in addition to being fun to work with. I have benefited both professionally and personally from my time at M3. My bosses have been generally supportive of my growth as a professional, although autonomy and self-evaluation are encouraged (if not an outright requirement).
Cons
Workload (and thus compensation and employment) is largely dependent on the mining industry. M3 has diversified somewhat, including specializing in the design and construction of large telescopes, but other non-mining work is not sufficient to sustain the entire body of the company. This, however, is not much different from lots of other industries (aviation/aerospace, petroleum, commercial construction). It's just something to be aware of if you're considering working for M3. 2016 was a rough year for mining, but 2017 appears to be looking better for companies like M3.