I think everyone i've met with the exceptions of a few people have been pretty cool and down to earth. Most of the time it's just for a quick "Hi/Can you take a picture/you were great tonight/etc.", but the ones that stood out:
Jon Howard from Threat Signal: Met him in 2007 at a Chimaira show in Toronto. Spoke to him for like 10 minutes. Talkative dude.
Brian Fair: He was near the merch table on the KsE tour in 2012 taking pics with fans. Just mention anything Boston sports-related to him and he'll talk.
Kevin Talley: This was in 2010 when Daath was touring with Chimaira. He was setting up Daath's sound stuff before the show started and recognized him. He tried to sell me DVDs of his Black Dahlia Murder and Slayer auditions.
Chris Adler: At a signing last October. Super nice guy, but I think everyone already knew that. And judging by his reaction, I think I was the first one to mention his work on the last Protest the Hero album.
Trevor Phipps and Ken Susi from Unearth: Susi's hilarious, and Trevor's a big Red Sox fan, so that was fun.
Brian Eschbach: I think he was just happy that I knew his Deadpool tattoo was of Deadpool, not Spiderman.
As far as ones that weren't really all that great, I haven't met anyone who was an outright asshole, but a couple who just seemed to show general indifference, a little annoyance by me saying "Hey, I enjoyed your set", or maybe even the subtle rockstar snub:
The dudes in Korn: I won a Meet & Greet pass during the Family Values tour in 2006. I chalk some of this up to their handlers who were very strict on the One item only and allowing you to take pictures of the members as they walk by, and not with them. So everything was rushed. Jonathan Davis didn't say one word, and just went by signing stuff. Munky was cool and talkative and actually the only one to seem genuinely happy and excited to meet fans. David spent the whole time talking to one of the security guys while he was walking up the line signing stuff. Fieldy was alright too, but seemed more focused on handing out bumper stickers of his clothing line or whatever. Considering they were my favourite band at the time and the first pro musicians I met that I liked, a bit of a bummer. Plus the Meet & Greet was scheduled during Stone Sour's set, instead of Dir En Grey's or Flyleaf's or something.
Alissa and one of the guitarists from the Agonist: Just genuinely indifferent to me saying "Hey, that was a nice set you played" and shaking their hand. Didn't ask for an autograph or picture or anything. In fact I think I was buying their CD from their merch booth when I approached their guitarist. Alissa in general just seems like an unpleasant person to be around at times.
09/19 - Crowbar/Revocation
10/06 - The Black Dahlia Murder
11/30 - Slipknot/Korn
12/27 - Protest the Hero