I'm sure you've heard the old joke about gamer college students using their fancy graphics cards to heat their rooms. :)

I did this last winter. Mining 24/7 on my personal rig noticeably took a bit of load off the heater and made a modest amount of ethereum. I stopped once the weather warmed as it seemed like a waste of energy.

I did the same, then my graphics card died and it was a net loss :-(

How were your electric bills?

Not appreciably different, that was the whole logic behind my thinking. In my case I have electric heaters, so doesn't make a difference compared to a gpu -- they're both effectively 100% thermally efficient.