Hiya!
In a nutshell, they are terrifying in my games. Basically, they are the fantasy equivalent of "A.I.". They don't think of themselves as 'individuals', have no sense of self-preservation past the most basic (re: don't walk off a cliff), do exactly what they are commanded to do, regardless of what it is or how it may affect others or pretty much anything.
A platoon of Modron's marching towards your city should be the equivalent of yelling "RUN!". Absolutely NOTHING you, or anyone else, can say is going to stop them from doing what they were commanded to do. There is no such thing as "diplomacy" once a decision on their part has been made. The only hope someone has to save their skin, city, country, etc, is to find out about the Modron's plans, then pre-emptively set up a meeting with the leader. That's about it. Once a Modron "collective" has made it's decision or gotten it's commands...that's it. No amount of "diplomacy" is going to work...they won't even acknowledge you in all likelihood; they will simply carry out their task or die trying.
Modrons: If they are acting already, you can't reason with them. They have their orders, and that's all that matters. Period.