Treesop. I walked out of the interview feeling terrible. I was accepted on Monday. As you know, we had been repeatedly told that the first half of the interview was going to be one-sided, and that the interviewers would be pretty stone-faced throughout that first portion. For me, aside from a few small smiles here and there, that part went as expected. (Although I must admit that one of my interviewers made a face of shock/horror when I discussed research regarding de novo in vitro synthesis of organs to help deal with the organ shortage (which really was an asinine facet of my answer, in hindsight))
In the second half of the interview, though, it was supposed to be conversational. Honestly, I don't think my interviewers said more than 50 words during the latter half. After conferring with other applicants, apparently theirs were far more conversational. Naturally (given my neurotic nature), I began worrying that I had COMPLETELY screwed up. I thought to myself "was I too talkative?", "too obnoxious?", "a know-it-all?", "condescending?". It all ran through my head; compounded by the fact that all the other applicants looked a little worried on my behalf that my interview didn't really transition to conversation like theirs had.
Anyway, as an admitted applicant, clearly my fears were unnecessary. I'm sure you were fine. I don't really think there's a hard-and-fast metric to assess whether or not the interview was a success.
GOOD LUCK!