I saw Bon Jovi open up for Ratt in '85 or '86. Bon Jovi was supporting his second album (7800 Farenheit, I believe). It was right before Bon Jovi really broke open big.
Anyways, McNichol's Arena here in Denver was sold out for the show (17,000+ seats). Bon Jovi played for about 30-40 minutes before Ratt came on. When Ratt hit the stage, half the arena had cleared out.
I saw Metallica open for Ozzy after the release of Master of Puppets, and the place was full when Metallica started. I wouldn't say that anybody left when Ozzy came on, but it seemed that more people where there for Metallica than for Ozzy.
I remember in 1985 we refused to see Bon Jovi they were the opening act for the Scorps. So we came late to the Garden to avoid Bon Jovi. I still hate that ass.