Letterman spent about 25 minutes (not counting commercials) with McCain, braced him about the issues, challenged his answers, never let it go beyond the bounds of civility -- and gave McCain time to make his points. On the other side, if we had seen more of this McCain on the campaign trail and in the debates, the current polls might be telling a different story. He was personable, he was generally funny (liked the throwaway that because of U.S. intelligence-gathering "we know a lot of [Letterman's] conversations") and he still delivered his message. If only more political interviews were like this.
One side note: When McCain came onstage, Paul Shaffer and the band played "Can't Explain." So many possible meanings there.