Hot? yes, but apparently cooler than England right now as long as you're a meter below ground in a ring around the poles (how's a constant 21c/70f sound)
this guy's website is so weird but I love the novelty of the idea and he portrays a plausible scenario for it being ultimately more habitable than Mars
Dumb rock doesn't even have a magnetic field. Even on Earth we're afraid of the destructive potential of cosmic rays and such, I can't believe what they'd do to us on Mars.