I have only been to the Onyx once, to see Hedwig and the Angry Inch. I had been meaning to check out a play at the Onyx for the longest time and decided Hedwig was a good place to start. They only had VIP tickets available, which are just front row seats. Lord, if there's a show you don't want to be up front at, it's Hedwig! She can be mean! Luckily my friend and I were not singled out during the performance (she laughed at a hipster with purple hair and spit Haribo gummies at another person).
I would say that of all the local theatres in the city, the Onyx is the one that puts on the most original plays. They are gutsy and provocative and maybe a bit politically incorrect. I'm a dork for not catching more performances but hope to see more shows soon.