My first year, I mailed them and felt like I didn't leave a connection for a response. This year, I e-mailed my thank yous. I also wondered if it was worth it, and maybe it's not. But, like my husband says, it could only help you. I mean, really, a few minutes of your time for the 20-30 minutes they spent with you. If they ignore it, so what. If they expect it, then you're in trouble if you didn't send it. It also gives you an opportunity to spark their memory. Good luck!
Oh, I got 1 call back before my thank you went through (what's up with the mailed call-back letters?). no offers, though.