I've generally had really good experiences meeting "rock stars", and I've had the good fortune to meet quite a few over the years doing what I do.
The ones that surprised me:
Dave Mustaine - everyone knows the reputation, but he was nothing but awesome to us, especially when he wasn't putting on "that guy from that band" public face. The dedication he had to doing what he does was really inspiring - he simply did NOT stop all day and gave a lot back to the fans. Obviously the guy has his demons and no doubt people would catch him on days where he'd be a complete asshole, but we never had any of those experiences at all.
Mikkey Dee - I'd heard rumours he was a complete cock. In fact, he was a complete legend.
Twisted Sister - top guys! Again, you hear that they're difficult to deal with (Dee and JJ especially) but they were very generous with their time and spent ages just hanging out and chatting to us all.
The ones that didn't surprise me:
Yngwie Malmsteen - yes. Interesting guy. What a welcome to the world of international touring artist support slots this guy was. Let's say his reputation was left most definitely untarnished after the tour we did with him...
Sebastian Bach - this guy is forever gonna be a 17yo kid who just loves to party and think he's pretty hot stuff. Not that there's anything wrong with that, but it tends to get old for people around him after a while.
Dio (and band) - didn't surprise me in a bad way, in fact I expected them all to be awesome and indeed they were.
I think at the end of the day, most of these people are... well... people. Everyone has their shit days and shit moments when you catch them at the wrong time. For all you know the person may be a lovely guy who has just had an hour of hell from a promoter and you've gone to get an autograph from them and they've torn your head off, so you walk away thinking they're a cock when they're actually usually pretty cool. It happens. And then you have people like Axl Rose...