This time of year I take the computer running my home NAS and move it to my bedroom and set up BOINC. Literally keeps the room 7-10 degrees (F) warmer.
Unless of course you have a heat pump (or an A/C with a reversing valve, which is the same thing). Which by all means please use that instead. You're not going to top a heat pump in terms of efficiency. We're talking 300% efficient heat production minimum. Best you can hope for with electric heating is 100%.
Well then you have me, I waited until winter to rip my DVD collection to my NAS, because if I was going to spend several weeks with 2 or 3 PCs blasting at 100% CPU, I might as well wait until it's furnace season rather than air conditioner season.
Pretty sure it's 100% efficient (100% energy consumed is emitted as heat) meaning it's exactly as efficient as a space heater. Only way to get more efficient is a heat pump.