Nothing like waiting out the storm. People have been downgrading using the alpha version of the hack, then update back to 10.3 because nothing is downgraded properly.
Haven't seen a brick though. Yet.
JustPingo and Mrrraou confirmed at least 8. I won't touch this "unofficial" with even a 10-foot pole.
By the way, I won't create a new topic for the downgrading progress, since, well, it's a very grey area that's not really fit for general GAF consumption. But I think that once the dust settles and the "official" tools are released (KTM, MCH2, etc.), the OP of this topic should be thoroughly revised. Easy, painless and safe downgrading will be a massive paradigm shift and it will render this topic completely outdated.