I use a synthetic 0 degree mummy bag for the winter. It gets cold sometimes if you are alone in your tent, but if you have people with you it's not that bad. I use a thermarest pro-lite inflatable because I like the packing size. However, I took the padding out of my pack and cut a 3/4 length 1/4" closed cell foam bivy pad and use it for the padding in my pack. I find that using the inflatable in tandem with the foam bivy pad that I have with me all the time anyway works quite well. I've used the RidgeRests and like them, but since I already had the inflatable, I found other things to spend my money on.
The 0 degree synthetic mummy bag works great for most cold weather as would a down bag as long as you keep it dry. A balaclava or hat is a must. Also, I get undressed in the bag and usually keep any extra clothing in there with me to cut down further on the air inside the bag. Chemical handwarmers that you can buy at just about any highway rest stop or gas station are cheap, small, and nice for the bottom of your bag if it's cold or in between two socks if it's REALLY cold. My feet are usually what get cold enough to bother me.
I have double bagged it a couple of times, but if I'm in a decent tent with another person or two I usually end up getting too warm. You just have to wait through the cold for a little while while everything warms up. A dog will help too if you don't have a person, their body temp is around 101F I think. They're little heaters and don't mind laying on your feet
A fleece insert or wool blanket is great for the inside of the bag if it's terribly cold. Santa just brought me a 40 degree bag for christmas that packs down substantially smaller than a fleece blanket (I can get it inside a nalgene). I'm actually anxious to try that as a liner, but it just hasn't been cold enough yet.
The only kind of heater I've ever used in a tent was a foil wrapped candle that we jury rigged from the tie points for the gear loft in the tent. I know, open flame, tent, etc. The tent was vented well enough and at the right distance it didn't do anything to the tent and made us feel warmer, although it's gain may have been marginal because there were three bodies in the tent. It also could be my imagination but I think it DID help in cutting down the condensation that freezes on the inside of the tent when you have three people breathing inside. Not to mention offered us some pleasant light to play cards by.
I guess as far as heaters go, I've been around them in unheated bunkhouses in the winter, and they are wonderful, but they were all electric. I guess if you are car camping and want to lug all that stuff around you have tons of options. I can't imagine even considering it otherwise. I hadn't considered the moisture that one of those propane heaters could produce. Certainly something to think about. I hate condensation on the inside of a tent, frozen or otherwise. It's a constant battle...