I had a birth plan, borrowed from a few different sources and meshed to reflect my values and wishes. It was read by all midwives at the FBC, my DP knew what was in it and it was carried out beautifully without me needing to cite it when I went in, in labour. The only bit that didn't go to plan was the third stage, which had to be managed due to some PPH. The attending midwife apologised profusely about this, but there was no need to do that because I know they had respected me the whole way through.
Not a waste of time, especially if you birth plan correlates strongly with the values and procedures of where you are giving birth. In my case, my birth plan was often just a rephrasing of their own mission statement!
I felt a lot of ownership over the birth we had because of the birth plan and finding a place to birth that was consistent with my birthing ideals.