I use it frequently. Sometimes you can get tickets for weeks in advance at other times they go on sale the day before. It all depends. Remain flexible.
I bought a membership in late 2011 and didn't end up using it until last summer, when I bought tickets for Sister Act and Anything Goes. One was the matinee, the other was the evening.
Both were reasonable prices, low $40's I believe, and the seats were great, Row C in the mezz off-center for Sister Act, Row K or L I think for Anything Goes in the orchestra, off-center on the aisle.
Very good investment, I got great seats for a good price and I'm kicking myself for not using it more.
I saw Ann tonight with tdf tickets. Orch H507. Last Saturday I saw Cinderella with tdf tickets and the seats were second to last row rear mezz. More often than not the seats are great and even if they aren't the investment is worth it.