My experience, I have related this several times in our forum talks, so you may be getting sick of the story, but I am a rambler and a'ramblin' I will go.
Last year (2006) I went through the season with 8 colonies (or so, can't quite remember exact number). I lost all of them except for one because of neglect with varroa treatments (and swarming too).
I had one little tiny Carniolan hive that went into winter, I didn't even think it would make it. Upon reading posts on the forum by Finsky about his use of a terrarium heater in the hives to help with weaker colonies come through the winter, I purchased one. Approximately $35.00. So....money spent, that is OK, I am an experimentor.
I gave this colony this 15 watt heater placed between the screened bottom board and the solid bottom board. I was using a screened bottom board that is called "varroa nator", a plastic screen that sits over the conventional solid bottom board.
This colony came through the winter. It was a colony that I really did not think had a hope in blazes of coming through alive, so I was going to try anything.
THis is the colony that grew like gangbusters this season. It was my colony that I had to make a cut down split from, (probably should have split it several more times), it was the colony that swarmed, I caught that swarm (on September 6, unusual for such a late swarm) and they are still going gangbusters. This swarm that came from my Carniolan colony even had a frame of capped brood the last time I looked, which was about 1 week ago. All the other colonies had absolutely no brood whatsoever and the brood nest was backfulled with sugar syrup. Still going gangbusters!!!!
I cannot praise use of the extra heat on a colony that may need some extra warmth enough. Have a wonderful and beautiful day, in our great life. Cindi