My heating (natural gas) bill for 2013 winter was reduced about 2/3 overall with last winter mining, while I don't have exact KW stats on hand, I'd estimate last winter I consumed about 4 to 5 KW running 24x7.
This winter I will consume ~6.5KW/hr which comes out to ~22K BTU's/hr. Still not enough to heat my home, but I guarantee it makes a good dent in my gas bill this winter as it's a 24x7 source of heat. Currently this week it's been in the lower 40's to lower 60's, a few windows open, and home is comfortable in the lower 70's. Will have to close all the windows once it starts to stay in the 30s-40's.
Michigan winter gas bill:
2012 mild winter ~$3,500 (No Miners)
2013 Horrible Winter ~$1,500 (miners)
2014 Winter TBD TBD

(miners)