Following a radio failure, got home into a class D airspace quite happily using an ICOM portable. No external aerial, but I did have the headset adapter, which I reckon is pretty much essential to avoid cockpit noise. P2 was able to hold the unit near the window, which I guess helped - occasional loss of contact while orbiting (in a Cessna) each time the low wing got between us and home!
I tend to think total electrical failure is more likely than simple radio failure - at least I've experienced two alternator failures so far, but only one radio failure.
While I sometimes carry my handheld GPS, I always carry the portable radio.