I had a FP for Wishes for our trip in August but swapped it out for BTM. During our stay though, I sort of scouted out the area to see if it was really worth a FP. I came to the conclusion that if you want to watch the fireworks without the hassle of getting a spot early and then maybe worry about people encroaching, I would do it. I also believe, and only IMO, that if you're with a group of say 4 or more people, I think it's really worth it because you'll be together and have a pretty decent view.
We got really lucky this past trip though. On one of our days, it rained cats and dogs all afternoon and most of the evening until about 9:20. We were in Magic Kingdom and although we weren't going to watch the fireworks, we noticed that it was very light in the way of crowds. We stuck around and watched the fireworks from the closest spot to the castle on Main Street that we've ever seen it. There was no one around us.... Obviously we got lucky but having swapped out my FP for Wishes a few nights earlier, I was kind of regretting it. This made up for it ten-fold.
Bottom line, I think it's worth the FP.