I always used to send hand-written thank-you notes, but when I started doing on-campus recruiting I realized that sometimes snail mail isn't feasible. The places I was interviewing were handing out second-round interviews the night of the interview or the next day, so if I wanted my message to reach my interviewer before s/he made a decision, I had to turn to email. Also, this is anecdotal, of course, but the only place I interviewed where I did not get an offer was the one I forgot to send thank-you emails to (I was traveling the whole day and then remembered too late...). Take that as you will! I personally think thank-you notes can only help you if they're well written and sincere.

The main reason to chose snail mail is not that it is slow, but that it shows some amount courtesy and appreciation.The cost (in terms of time) of writing an email v/s letter should not be measured in terms of the number of minutes. I think of it this way, somebody who had take the time to get some postal envelops, stamps, sat down and wrote the letter leaving his work in some way and then dropping it off to a post box is much more classy than someone who composed an email in his/her already open client and wrote a bare bone email with a template in their drafts folder.

When you talk about the time it takes to compose and send a physical letter, I simply don't agree or understand. How does taking more time mean you're more thoughtful, serious, or classical? If you're just saying that snail mail seems classy because of its cultural significance, then I understand, but just taking a long time alone doesn't seem particularly thoughtful, serious, or classy.

think of it this way, I could be doing something else while I was composing your letter. Also hand written things are always much more personal than email.