I've worn out a couple kitchen type electric knives cutting foam-but that was in a commercial setting, where we cut a lot of foam, every day. We bought them at a 2nd hand store, so they had been used previously.
I've also had good luck with the wavy jigsaw blades Bosch makes for soft materials.
I don't like hot wire cutters for foam, though due to the fumes as the material melts.
Hope this helps.