Being a Canuck, I LOVE the REST feature. In the winter, it circulates the coolant through the heater core, and runs the fan at a low setting. It runs for 30min or shuts off it battery voltage gets low.
In the summer, tilt open the sunroof and set the temp to"LO" and use REST to circulate a little cool air through the car. Not nearly as effective as winter use, but does help a little.
1998 C230 "Black Betty"