I wrote birth plans for my 3 births. They were read by all midwives when I went into labour and I was lucky that they could be followed out.
With #1 I had said I didn't want an epidural unless I really wasn't coping. At abot 7cm dilated I asked whether this meant I was too far along for an epidural (I was testing the iodea out iykwim) and the midwife gently encouraged me away from the idea. I had also said on my birth plan that I was a private person who hated losing control, and this really set the tone for how I was treated.
My last 2 were HB's and the first thing the midwife did with #2 when she arrived was read the plan. She then sat back and just watched me as the baby arrived 10 mins later. With #3 my midwife knew me from old, read the plan, laughed and said she wanted a copy as it was very short, funny and to the point (ie don't examine me unless a problem, leave me to it, I will let you know when I need anything).
I think birth plans are useful, but can't always be followed. I found it most helpful to say what I was like, as it is hard in labour to say - 'shut up! I want quiet' or 'I hate being talked about' for example. So, I said I wanted my births to be as quiet and calm as possible, with as little fuss as possible. I also said I felt patronised quite easily, and it made me angry lol!
Birth plans also helpful about things like Vit A and whether or not you object to students being present, or whether you want a physiological 3rd stage. As to what sort of pain relief, often you don't know till you are there, so best to keep your options open on that. They are also helpful in that they make you think about the possible options and outcomes so you are well informed about things that might happen.
Best of luck!