Dear ladies and gents, I returned a couple of days ago from Paris and would like to share my experience at the FSH flagship. Unfortunately, we were not offered a B or K, but hopefully this information will be helpful to some who plan to try for a bag.
My two daughters and I arrived on Saturday the 17th on different flights with varied arrival times, as we all live and work in different US cities - we had planned a fun girls' trip to Paris. None of us were able to get an appointment for Monday. I did not realize that many boutiques are now open on Sundays, so we ended up shopping for RTW and shoes at Givenchy and Chanel after some museum time. Monday we had our Hermès museum tour - it was fantastic. I don't think we will ever look at Hermès with the same eyes again after seeing the beautiful and varied artifacts, books and objects d'art that are the inspiration for the brand. We did do some shopping that afternoon- some silk, bangles and a scarf ring. Perhaps because it was in advance of fashion week, all of the items that I had planned to purchase (RTW, boots, jewelry) were not available in my sizes

One daughter and I paid for the items on two separate tickets. We tried again for Tuesday appointments- the girls obtained them but I did not.
Tuesday morning we arrived at FSH for the first appointment at 10:30. There was a line waiting to get in and as soon as the doors opened there was a rush upstairs. Two couples were turned away by the leather hostess because they didn't have appointments. They were rather angry and rude with her and arguing that they should be seen, but ultimately left. My younger daughter was introduced to her SA, and chatted for a bit. She then gave him her wishlist: K 32, B30 or B35, neutral preferred but also likes green or dark blue. He told her it was very difficult to obtain the bags in the specs that she wanted, but he went back to check. Meanwhile, several other clients were being told "I'm sorry madame, but no Birkin or Kelly delivery today" and were being shown Garden Party and Evelyn bags. At this time my elder daughter received her notification that the SA was ready - about 1 hour before her appointment. and she went off with him to the housewares department. In the meantime, the first SA returned and asked my daughter to accompany him to a private room. Naturally, I went with her! She was offered a Rose Azalée Constance 24 in Epsom with gold hardware. While it was a difficult decision, she turned the bag down, as it didn't fit her style/wardrobe. She was also shown a malachite CdC key-chain, which she also declined. We thanked the SA and left to find her sister.
Sis also was with another delightful SA, but again no Birkin or Kelly. He offered her a very bright pink Lindy, an etoupe Bolide, and a Jypsière. It was a close call with the Jypsière, but ultimately she decided it really was too casual for her work needs. We tried but did not receive any other appointments during our stay in Paris. When we return however, we will try again.
I believe it is a lottery for an appointment. Obviously, the regular FSH clients are not part of this system - I'm just referring to the walk-ins like us. As other posters have mentioned above, a large spend may help you to be offered a coveted bag, but there are exceptions. My daughter bought nothing at FSH, had no purchase history with Hermès in Europe and yet was offered the Constance. Part of that offer came from the fact that she and the SA seemed to get along quite well. I also think that since the process is so iffy for those who are NOT regular customers it is best to work with your own SA at your home boutique, at least in the US. I realize that there are other countries where it is so difficult to purchase a B, K or C that Paris is really the only option, but for others, it's probably easiest to work through the home boutique.
Thank you for listening to my story, best of luck to all on the hunt in Paris and congratulations to all who were able to purchase their desired bags!

And remember: Paris is always a good idea!

(Thank you Audrey Hepburn)