Re: What exactly is Regeneration like to the Doctor?
I have wondered about that too. The regeneration process has been shown to be different for each doctor, but in the new series each subsequent iteration seems to become more violent and unpredictable. Given how Tennant went out I wouldn't be surprised if Smith's bowing out causes a nuclear detonation. That said, I imagine there's probably a great deal of pain and fear involved. Is it like death? Hard to say.
"Two possibilities exist: Either we are alone in the Universe or we are not. Both are equally terrifying."
--Arthur C. Clarke