Back when I used to pay for it, I would consider that I got value after 10 visits. It worked out roughly $50 a visit. (My best year of flying I got that down to $10 a visit, pity short haul red-e-deals don't give much in the way of SC's).
Of course not all visits are equal, I would not consider $50 for a glass of OJ, a coffee and some pancakes to be good value, then again $50 to be knocking back drink after drink, and a full sit down meal at HKG lounge was pretty good value, especially when I had say 3 or 4 hours to kill, so together the two evened each other out, even with a small number.
So would I spend $39 on a single visit, knowing that there where no other visits to other QP's / JLounges to help even things out? No.
For $39 you can get quite a reasonable dinner down stairs, and still have change left over, even after paying airport prices. The SYD QP is not going to be quiet on a Friday night, it's going to be very ordinary food options (I typically don't eat in QP's), and how many drinks can you knock back in an hour and still be allowed onto the plane?
Unless you want to see what it's like upstairs (and I totally understand, I remember standing outside those glass doors wondering how the other half fly), save your money.