I tried using Shettles' book, and it didn't work for us. The first time we were trying I wanted a girl. So, we had sex about 3 - 4 days before I ovulated, and then I panicked that DH's sperm wouldn't live that long, and we had sex the day I ovulated. I was sure that last time would result in a boy, but we had a girl. Maybe from the previous sex there some girly sperm still hanging around that got to my egg first, who knows.
The second time, I really wanted another girl. We had sex what I thought was 3 - 4 days before O, and then as I continued to chart, I realized it was actually 5 or so days. I thought there was no way I'd get pregnant that month, and when I did, I was sure it was a girl. But, it was our son.
My cycles are very regular, and my charts are crystal clear about when I ovulate, etc.,. We have been use FAM to both avoid and acheive pregnancy for 8 years. But I don't truly believe that timing has much to do with gender.
When I looked at a website about Shettles, it mentioned a different fertility expert (Elizabeth Whelan) who believes the exact opposite of Shettles, and that having sex closer to O results in girls. So, who knows? It's a 50-50 shot.