i tend to find it a put off, sucking on a guy who is up for it but cant get hard is a big turnoff for me (happend earlier today with an ex actually) eventually with a bit of persuasion he got hard, but it does ruin the mood somewhat.
I dont mind if a guy is soft once im up him however, as long as he is moaning (and they do, like bit""es) and i know hes into it and begging for more. In fact in some cases i find it better as if they are on top because i feel there nuts and cock bouncing against me which makes me work harder