I've got to see meet Koening twice, and Takei and Uhuru once. I saw them at all different TrekFests in Iowa, which is a pretty low-key event.
I was really impressed by Koening. He seemed the most grounded in reality of the celebrities that I've met, and the least like fame had changed him. Very warm and friendly; quite laidback. This was before his recent personal loss, though. The first time I saw him I told him I thought it was cool that he was willing to do fan-films, like the Continuing Voyages, and he seemed pleased.
George Takei seems to be the kind of person who loves meeting new people above all else. He would get excited talking to people signing autographs, and his handler would have to remind him that the line was long. You could see the disappointment in his eyes -- he really would've kept talking with all of the random fans if he could've.
I don't know if Uhuru was having a bad day when I saw her, but she seemed more interested in herself than Star Trek or the fans. At the speaking event she went on a ten/fifteen minute spree talking about herself, and Koening eventually interrupted her, saying this isn't what the fans came here to see and hear. Which left things kind of awkward -- who wants to see their favorite stars argue with each others? She also wasn't in the autograph mood that day; didn't say a word to me.