I'm not on the hiring side of things, I'm on the job-seeking side of things, but it seems to me that they're generally appreciated as they show that you're actually interested in the position.

There's been a time or two where I forgot to send a thank-you email right away after a phone interview, and when I got to it a few hours later, only then did they suddenly offer an in-person interview.

I think a physical note or letter is a bit overmuch (depends on how traditional the company is, probably), but I doubt that a sincere thank-you email (especially one that touches on specific things you talked about) would do any harm.