Anyone with experience in using the cabin heat in below freezing temps. In other words, how efficient is the system.

Best heater in any single engine I've experienced. If you don't have many air leaks in the cabin by the time you level off for cruise you'll be pushing the heater knob in as your pulling the throttle back

I have had the opposite experience of VA Maule. It depends on the aircraft. I have instructed in dozens of different Maules. Some the heat worked but in most it was minimal. I didn't take the cowling off to analyze the situation in each case. I can tell you there are ways to improve the situation. After a few modifications I can fly at -20F and have enough heat to fly without gloves.

Lots of heat for me. I have aux/extra heater which has the left exhaust pipe cuff's heat directed at the pilot's feet and the right muffler's cuff to the floor vent under the right seat aiming towards the rear. We melted peanut butter that was on the floor in the back while on the way home from Alaska in cold temps. Even through Montana. Nice and warm as long as all the drafty areas are sealed up enough.

Why are some heaters good and others not? Just returned from Maine at 6000; -10 F. Froze. Needed four layers of clothes. Shop said heater was mechanically working fine. Am I missing something

Sorry you're having poor heating, few things to look at.
Is the sliding damper valve fully opening? Take tube off to look if it's fully opens by the bowden cable, if the SCAT tube is too short it won't let it open fully ether.
Do you have tears in SCAT tubes?
Is there an obstruction in SCAT tubes ( mouse nest)?
Is the shroud in good shape around the muffler, they have a tendency to tear & crack.
Are the hot & cold SCAT tubes swapped on the sliding damper valves ( don't any of y'all smart A$$es ask me how I could know that something like this could happen)

thanks for the advise. I will check as suggested. feels good to back home in Georgia.

Did you get your heat working any better?

It appears to work fine...just can’t handle 10 below. Sealing air leaks key

on the 540 powered maules, i have found the f. atlee dodge mufflers produce much better heat than any other brand. they have larger diameter tubes to bring hot exhaust gasses in quicker, and they have dozens of heatsinks welded to the outside of the muffler can to help dissipate that heat into the heater muff and ultimately the cabin.
flew home from the cabin at -35 a couple weeks ago and I didn't hear any complaints from momma, 6 year old boy, or his dog. the defroster was struggling to give me 2 little holes to look through, but after 10 minutes airborn it cleared up.
trying to talk the engine into starting at those temps, after 3 days at the cabin- that's a different story