Stuart, you're right the rail connection will tempt Yorkshire folk, but the Anytime and Offpeak fares from Sheffield are no cheaper than to MAN. Journey times once you include the bus are likely to be similar to Sheffield -> MAN and unlike MAN the first trains of the day aren't early enough to safely make any flight before 8am.
As for DSA, well it would be nice if an airport that carries Sheffield's name had any direct bus or train link with the city
