Re: INterview where you knew you did not want the job
What you did was indeed honest, however, the more interviews you're exposed to, the better groomed you are for the one around the corner. I'd rather complete the interview and then let them know. Knowing that the job is not for you makes you a bit more comfortable since you have nothing to loose but the time you had already set aside for it. I am sure the interviewers would have done the same and would only gain from interviewing you.
This is just my opinion though.