I was impressed with him, but then again I expected to be because I am nearly always impressed by him. I like that he isn't hyperbolic, as many presidential candidates of past and present are; it makes the times he does come out swinging, even that much more savage. Just an observation in terms of delivery, I like how he doesn't tend to feed off of the crowd's energy when he's talking to someone. by that I mean he has this habit of making you forget there's an audience, pausing to allow the applause and hooplah to pass, while straight faced and waiting to finish his thought. His "I don't care"comment in response to Trump's tweets, was so impactful because while a natural urge would get a self-satisfied smirk at the audience's explosive reaction, he continued focusing on Wallace, and letting the words hang there in his expression, conveying even more so that he really doesn't care about Trump's insults, and wasn't just playing at being non chalant.
He has great natural instincts and while I don't know yet who I will support, he is the one I am most excited to see in the debates. I think he has a killer combination of great natural instincts and learned behavior (I can tell how much his philosophy studies have guided his interactions with people and his motivations).