I've been sending them to every person I interview with, the people that coordinated the weekend, and any grad students that were helpful. They are all thoughtful and personalized and I try to add something funny or unique from our conversation. I just set up all the drafts with the emails on the interview list in gmail and write something short unless it's someone I really connected with. Before internet I'd send thank you cards to people I interviewed with for jobs and drop it in the mail on my way home. It might be old school but I've always had positive results from it.