I thought Dion won (whatever the hell that term means in this context). He struck me as passionate but calm, and as someone who very much knew what he was talking about. (The 8:00 PM CBC Radio newscast suggested a quick poll of Quebecers agreed with me.)
Harper struck me as little more than smug (but note that I despise the man nearly as much as I loathe his policies), occasionally defensive but always convinced not so much that he was right, but that he was better than those he was debating.
Layton impressed me more than I thought he would (and I speak as someone who has almost always — though more often than not, with nose plugged — voted for the NDP). His French was very good and he too seemed to know what he was talking about, though his robot-like insistence on bringing every question around to "families" and ordinary people also makes me cringe as sounding contemptuous.
May impressed me largely because I had heard that her French was really, really questionable, and it was clear she understood what was going on around her; when she stumbled, you knew it was because she was having problems with vocabulary, not because she was retreating to sound-bites because she had nothing else to say.
And Duceppe? Well hell, he's the eminence grise of these debates. He tore into Harper with a will and I suspect he did a lot of good for the (temporary) future of the Bloc.