well, I've a Hawk Shell and no factory heater. Sometimes I wish I did when it is really cold and the temp is below freezing. I do like the fact that for the summer, I don't take the big propane tank and have lots more room for my junk. In fall, winter, spring, i take the little buddy. I only use it when I am awake so that the risk of fire and all is lower. Lots of down stuff for the times that it is off. i don't have a cold insulation setup, but that I do wish I had. May make one one day.
I also have a small ceramic electric heater that I use if I just happen to have access to AC power. very small, works well and no propane. I use it when camped outside a friends house, or a state park with hookups during the cold seasons. If I know I will have no access to AC power, then I just leave it at home.
If the temp is really really low, like below 20, sometimes i will keep the top down, and sleep just on the fold out couch. Thicker padding, warmer, and very cozy. Not a lot of stretching room, but it is a bit warmer.
Good luck.
Dave in Seattle