My uninformed opinion on the chancellor's debate: Darling did reasonably well from the weakest position, but the fact he had a very difficult job to do doesn't change the fact that I don't think he did it. Osborne was worse, I think; I don't think he did much to differentiate his position from Darling's, and kept trying to interrupt in a way I found displeasing. On the other hand, he might have done well in frequently agreeing with Cable. There are far worse tactics than to imply Darling is wrong and Cable is right but his party unelectable (though the dominant opinion of my sample of four was that it was a mistake to make that point explicitly).
I'd say that Cable was the comfortable winner, actually. He stayed out of Darling and Osborne's squabbling, reminded everyone that his party had been far better at predicting the financial crisis than anyone else, and offered a nice little zinger regarding Osborne's plans for the "death tax" (I wasn't convinced by his delivery of it, but both my fellow viewers and the audience significantly disagree). His "experience" argument seemed pretty compelling.
But hey, I appear to have ranked the three competitors in the exact order I want them to get/retain the job! What a coincidence! Anyone out there got a different take?