Two of the three HIX's near the Madrid airport are walkable from metro stations. The third, the so-called "Airport" HIX, is accessible only by an expensive taxi or shuttle ride, and has been negatively reviewed here and in hotel review sites for that precise reason.

I stayed two nights in the "San Sebastian" HIX last November under the points breaks deal. It is within walking distance of the "Hospital Infanta Sofia" metro stop in a recently-developed industrial neighborhood. It was fine, and has a nice (nice in sense that one would be comfortable bringing a date along) Spanish-food restaurant around the corner for the local weekday workers. This HIX is an hour by metro from either downtown or the airport. Coming from either direction, you transfer or go thru the "Nuevo Ministerios" metro to Infanta Sofia end-of-the line metro stop. Exit in front of the hospital, walk to the gas station, look down the intersecting street in the airport direction, and one can see the HIX another block away. Fare is 2 euros.

Personally, I rather stay at this out-of-the way HIX, than pay three times as much for the Holiday Inn in the center of Madrid. Location is money apparentally. I noticed the same trade-off in a Frankfurt HIX in a bleak industrial neighborhood, that was actually very accessible by bus. Having a workable public transit option makes all the difference.