As an interviewer, I’ve gotten a few thank you notes that have made me stop and rethink a decision. It’s always nice to get a well thought out, personal thank you note. In talking to other interviewers, many have said that a post-card or handwritten letter really makes them feel special. You will have to gauge how you think your interviewer would feel about a handwritten note.
Believe it or not, thank you notes can sometimes hurt an applicant. I once got a thank you note before the day was even out that was clearly generic. It reflected poorly on the applicant. I also once got one that was full of typos. Again, a poor reflection on the applicant.
MedSchoolCoach recommends the following approach: a short, but personal, email to your interviewers about 48 hours after your interview. Include something you talked about, something you laughed about or connected over. Do not include your grades or your resume. Do not write a thesis and try to avoid saying that school x is your first choice (you can’t really say this 48 hours out can you?). Keep it simple and professional, but personal. If you feel like your interviewer would be one that responds positively to a hand written note or card, than by all means go for it as well, but make sure its legible!