I'm thinking if you live somewhere cold, it's definitely worth it during wintertime! That way the electricity you're burning off (probably at a loss) warms your toes.

Just for fun, I have a Mac Pro with its stock GPU cranking away a massive 5 Mh/s... I joined a pool and it looks like after a week I've earned one bitcoin dime! (.01 BTC)

But boy are my toes warm.