I used it only once, like Janet, when I showed it to my boss and this led directly to my transition to full time, name and ID change, the works. It was the best move I ever made.
But I'd been too timid to speak up about it. My hand was forced when I was reported to have been seen in a women's room on a different floor in the building where I worked. I had decided a month before that I would never set foot in a men's room again. My boss was informed of this incident by building security, and when he asked me about it, I handed him the letter, and next thing I knew, management was pushing me to transition ASAP. They were very supportive, and what a tremendous relief it was!
I have been using women's rooms for the past 4 years. When wearing a skirt and heels and makeup I have never had the least problem, and these days I pass well. But the time I got clocked at work, it was because I was wearing pants and minimal makeup.
Now practically every trans woman will tell you that in order to pass you should avoid skirts and makeup, just wear pants. For me, I found the contrary to be true. Maybe it helps others to dress down and not appear feminine, I don't know. But in my case, my inherent preference for femininity is also what allows me to pass and live my life full time. Outside of work, I've always gone "high femme," it was just that I wasn't out yet at work, even though I'd promised myself never to set foot in a men's room again. After one month of using the women's room every day at work, I got clocked and reported.
So that was the only time I needed the letter, and it served its purpose. Now the therapist who wrote it is retired, and there's no more need for the letter. My driver's license has that sweet, sweet F, which is way better.