My brother read this post over my shoulder, asked what it was about, and when I explained, he replied "No." I started to give the various arguments for finishing the series, and was just repeatedly interrupted with "No. No. No. Nope. No."
I suspect this will be the general reaction of fans of the series.
I also suspect that even if the sixth book is a decent piece, it will never be treated as part of Hitchhikers, just an addition. Glorified fan fiction, given more hype and controversy than it probably will deserve.
But I'm not going to lie, I'll most likely read it anyway.