I understood but was dismayed by Obama's passivity in the first debate.
So when he started looking Romney in the eye and saying "That's not true," I liked it a lot.
I didn't like the interrupting as much, but I also understood that the fact that he was trying to be calm and statesmanlike in the first debate while Romney ran roughshod over the moderator helped get things off to a bad start. There didn't seem to be many options to both get equal time and not interrupt.
Later he became more comfortable with it and I think interrupted a bit too much in the last debate, when Romney was trying for the statesman thing and so it looked bad by comparison. (Though Romney did still interrupt, just much less than in the first two debates.)
Second debate was just about right.