Cool dudes/bands i've met and/or friends of mine played/toured with and told me about, really nice, down-to-earth and open for discussion (i'm not the "let's smoke weed and get drunk" type, i like to talk and socialize) :
Misery Index/Dying Fetus : all of them (esp. Jason)
Decapitated
Mitts from Madball/Skarhead/RagMen/H20
Merauder
Matt from Skarhead/Subzero/Merauder
Death Before Dishonor
Gojira
Knuckledust
Daath
Suffocation
Animosity (RIP)
Hoods
Hatesphere (Jacob B mainly)
Immolation
Malevolent Creation
Pofony(RIP)/Medeïa -> Finns are nice, period
Tom Gardiner from Hateform/Scorched Earth Tactics (RIP) -> Finn -> nice
Eyeless/Weaksaw (but those are my buddies anyway)
Angel Crew / Backfire
Billy from Biohazard/Blood for blood
Providence
L'esprit du clan
Benighted
Aborted
Trepalium
Gorod (esp. their drummer Sam Santiago)
Hate Eternal
Never met but seem super chill judging from seeing them on stage and videos and internet contact : The Black Dahlia Murder, sick of it all, sepultura, exodus, swashbuckle, stone sour, bury your dead, ignite, trials, breakdown, all out war, sylosis, downset, a wilhelm scream, down, six ft. ditch, Terror, Gene Hoglan, The Acacia Strain, Napalm Death, first blood...
Still, now i understand how some bands would not seem very social, when actually they just want some quiet/rest before/after the show (referring to the The Haunted comment above mainly).
Also, be aware that even if you're a "star" playing in front of huge crowds every night, it's also very possible that you're still a shy person and not super comfortable when strangers are coming to talk to you in a super-excited way.
Guys/bands that seemed like dicks to me at some point (judging from seeing them onstage / videos), but now in hindsight i think it was just "they are cool but were having a bad time/looking for some quiet when i saw them so they seemed like dicks" : chimaira, lamb of god, cannibal corpse, machine head, in flames...