I did not know this. Does the Elk M1G support Insteon?
Yes, very well. With an XSP unit, Elk directly controls and responds to Insteon. It is a little clunky however. If you wanted to supercharge your Elk system and have top notch integration, get the ISY unit and the Elk module for it. The ISY has vastly more programming space and flexibility and the Elk integration module for it is about 99% integrated. In other words, you could pretty much never write an Elk rule, you could do it all in ISY with a much nicer and vastly more powerful programming ability.
Zones: ISY is aware of status of all zones (normal, violated, bypass, touble, etc). Voltages by query.
Outputs: ISY has real time status and control of all of them
Keypads: ISY has real time monitoring of all keypad info including button presses and temp (and it can use that temp to set programs which native Elk rules can not). It also can send messages to the screen and turn function buttons on/off.
System Status: ISY knows the status of the system (ready to arm, force armable, not ready, alarming (burglar/fire/etc), and so forth.
System State: ISY can arm/disarm the system into any state.
Thermostats: I believe they are fully integrated (don't have one to test but the menu items are there)
Sound System: Same story as thermostatsSpeak: ISY can have Elk speak the same as Elk native rules.
What it doesn't do:
Tasks: Can not directly trigger a task. Requires using a "phantom" output as intermediary.
Counters: ISY has no way of knowing what a counter value might be. You would use ISY's own "counter" system.
Custom Settings: ISY doesn't know what they are set to. But you would never need them anyway as ISY has its own set of variables.