My experiences have been generally good.
My family and I met Joe Satriani at the San Fransisco airport in 1995, around the holidays. My brother and I approached Satriani and told him we liked his music. The guitar hero didn't even seem like a rock star. He was touring with his (rather plain looking) wife and infant daughter. He was very approachable and laid back, and he appreciated that my brother and I had previously been to one of his concerts in Portland, OR. It was funny the way my mom went up to him, shook his hand, and said, "Hi, I'm the boys' mother." Lol. My little brother played bass guitar, and was a big fan of Satriani's bassist, Stu Hamm. He even had Hamm's instructional video, and got his picture taken with Hamm.
I briefly met Scott Ian after a concert in downtown Portland in 1996. He was quick to sign some autographs. He was decent. Admittedly, he didn't really make eye contact with many of us. Ian was anxious to get to a bar across the street. My friend Mike commented, "he's too drunk." That got Ian's attention, to which he replied, "Oh, I'm not drunk yet." Funny guy.