Yes, I did see the original cast of Dreamgirls. I cut high school on a Wednesday in May of 1982, right after the Tony nominations came out, and saw a matinee. I got a first row rear mezz seat at the TKTS booth. I think I paid all of about $20 (maybe not even).
Jennifer Holliday had people whoopin' and hollerin' and shoutin' "sing it sistah" before the song was even over. I'd never had a Broadway experience like that before, and very few since.
I sat next to an old Jewish couple who had no idea what all the fuss was about. At the intermission, the man turned to his wife and said, "So...this is theatre?!?"