Well given that all but a tiny percentage of the energy consumed that’s not waste heat is either going into useful computing or keeping fans spinning (which you would have in any sort of HVAC solution), I’d say it’s super efficient.

That depends. The heat might be used in a heat pump for all we know.

Significantly cheaper though.

Commodity window-mount heat pumps which run off household power outlets can be had for a few hundred dollars, and are good for a minimum of a couple kilowatts of heat output.

Any computing solution which dumps a couple kilowatts of heat and didn’t fall off a truck would probably cost more than that.

Computers are fairly efficient heaters, but they’re a lot more expensive than heaters.

Cheaper to buy, but not to run