Write them. It'll make you feel better.
(In case you don't want to follow the link, here's the key half-quote: "...recent scientific findings linking gratitude to increased optimism, stress reduction and a better night’s sleep.")
That's not the reason to write them, of course. You write them to make the people who did something nice for you feel good. But feeling good has always been a by-product of thank you note writing for me. I feel good when I cross "thank you notes" off my to-do list - satisfaction in completing something. I feel good for the same reason when I put them in the mailbox. And I feel good later on, if I get a compliment on the note itself (those compliments used to be for my notes...now they're for Dixon's. And let's be honest, getting a seven year old boy to write thank you notes is, in fact, a gigantic accomplishment.)
I'm embarrassed to say that I'm a little more slack than I used to be in the thank you note department. I sometimes do send an email when I used to send a handwritten note. I shouldn't - this article was a good reminder that nothing replaces a personally written card.
Plus, email doesn't provide any excuse to buy new stationery. And...yes to new stationery.