I've had mostly good experiences with the celebs I've met.
Bridget Hoffman (Echidna from Hercules: The Legendary Journeys) was a complete sweetheart. The con I met her at, she was sitting at her table all alone, no line. No one recognized her, which isn't too surprising considering all the makeup she wore on the show. We talked quite a bit. (She also had
amazing legs. :love lol)
Terry Farrell was my Star Trek crush (gimmie a break, I was 14 :$) and I was nervous as Hell to meet her. I remember handing her an action figure to sign and asking "will you sign this?" I must have been stiff as a board because she looked at me with a big smile, totally stiffened up and said "yes!" I felt like a dork, but she was totally cool with it.
The best so far was Richard Herd. A friend and I were standing near the autograph area at this particular con, trying to figure out what we wanted to do next. Every few minutes we has someone asking us if we'd take their picture with Richard. After the 4th time, he looks at me and says "yanno what? You need a picture!" He took my camera from me, snagged some passer-by and asked them to snap a shot. It was awesome.
As for bad, Lawrence Montaigne was kind of a dick. I bought a photo from a dealer in the dealer's room, and took it to him to sign, not knowing he was selling them himself. When I handed it to him and found out he was charging for autographs (which was a new thing to me, since no other celebs had charged me for them before), I got a lecture on how buying from him would have gotten me a nice, crisp-colored picture instead of the "rip-off" I'd purchased elsewhere. Due to that, I was pretty indifferent. Polite, but indifferent.
I sat near Peter Mayhew in a hotel lobby a few years back, waiting to meet up with a friend. Never said anything to him, as I didn't want to potentially irritate, but listening to him talk to his handler, he was being a grump-ass. But I can't blame him too much, seeing as how he seemed exhausted, and with some of his health issues. It's good to hear he's a good guy if you catch him fresh.
The absolute
worst that springs to mind was running into "The Big Show" Paul Wight in the airport. Approached and asked for an autograph, and he was just a complete dick. My favorite part

rolleyes) was him telling me "Now don't follow me to my plane or anything!" Yeah. Complete ass.