I try to send a PM to the person when they give me karma. Just a quick thanks and maybe something else depending on what they wrote (if anything) in the comment box. I've gotten to know quite a few people at MR that way. To me it is a matter of respect, if they thought enough of what you did to give karma, acknowledge it and thank them for it.
As I remember it was UncleDuke that kicked off a whole new wave of karma (and "doughnut") giving about a year ago. There are days I give out no karma at all, there are other days that the system tells me that I have reached my limit for karma giving that day. (10 times in 24 hours.)
NatCh was right, it feels great to get karma from people just starting out that are thankful for your answers, it also feels great to get karma from people you respect. The short answer is that any karma feels great -- each in its own unique way.