Master explained that my entry into subspace was due to the release of endorphins in response to the pain and stresses my body experiences during our play. i guess that makes my Master pretty smart since it's nearly what your friend says too ( i always thought He was smart.
i have experienced that shivering affect as well as the feeling of euphoria. i also do feel rather spacey after being in subspace, sort of lost. my thought processes are impaired for a bit. But even while i feel not so bright, at the same time i feel very relaxed and refreshed.
for me it is an amazing experience, as one moment the pain is so intense i am debating using my safe word, and then in an instance i can see more then feel the pain and i seem to be enveloped in a feeling of peace and well being.
As you say, i also am sure the experience is different for everyone.
slave shaya