The problem I have with thank you notes - both as an interviewer and an interviewee - is that not only are they mechanical in nature but also manipulative. The interviewee has something to gain by sending a thank you note.

When all the people you have interviewed send you a thank you note because it's "the thing to do," then it is hard to gauge the level of sincerity.

For me, the enthusiasm and energy demonstrated at the actual interview is far more indicative of whether either party really actually appreciated spending time together.