When you get an invitation, do you respond, whether or not you're going? I've noticed that many people don't. (Has happened to myself as well as to friends.) To me, it seems the courteous thing to let someone know as soon as you know your plans, even if you have to say no. Am I expecting too much here? Does it matter whether you invited them by e-mail, on paper, or spoke to them by phone or in person? Is it just hard for some people to say no, so they'd rather avoid the issue altogether?

I'm curious: What do you do when you're the one getting the invitation, and how do you feel if you're the one doing the inviting and people don't respond?