Not a single thing on my birth plan (though I did call it preferences) came to pass, but I was really glad I had it as it made me think things through in advance.
For some of it, I was in too much pain to talk, so it was good DH knew what I would like to ask.
At one point, when they said we were moving to theatre to try a forceps delivery and do a c-section if that didn't work, I asked if they'd be able to try a ventouse delivery instead, and they paused and said 'well it might not work but if you're ok with being moved at that point, we'll give it one push and re-assess then.' And it did work, and so I was able to give birth vaginally without forceps which was a preference, and I don't think I'd have thought to ask if I hadn't thought it through in advance.
I think it's very fashionable to dismiss women for wanting control over the process, see the negative comments about candles and music. I listened to hypnobirthing tracks for the start of my labour and it really helped me stay in control, there are lots of lessons from sports psychology about how the body deals with pain and it's limits that can really help women having a normal healthy labour.
At almost every stage my 'option 1' didn't work out, so having looked at the evidence and thought about what I wanted really helped me in being able to ask for my option 2- in some cases, that wasn't possible, but where it was it was really good. I had an induction, an epidural, an assisted delivery and a third degree tear, none of which were things I wanted, but on the whole I feel positive about my birth because I felt I was respected and listened to by all the medical professionals involved. For me, having a plan helped with that.