I like the double Ms. It reduces the ambiguity. Nobody thinks of 10MM as anything other than 10 million. 10M can be 10 million or 10 thousand. I've been in situations where the ambiguity has caused problems.

For me (UK, not US) it increases it.

Nobody outside the financial sector uses it here, at all. And 10M is always understood to mean 10 million, never 10 thousand (that's 10k).

I hate the finance-style Roman-numeral notation. There's no good reason I've ever heard why to use it instead of metric-style notation (other than tradition).