Like at Six Flags for example? Well, living next to one I can tell you that it is worth it on select days.
If you're going on a holiday, you're going to want to buy one. Also, if you're planning on getting it, get it in the morning. They do sell out of them as they have a set number of fast passes that they can give out.
Whenever we go to Fright Fest, a big halloween party at the park, we get fast passes. If not, the lines would take 2 hours each. With the fast pass, you're in line for maybe 10 minutes at the most. It's definitely worth it.
It might also be worth it on the weekends. If it's a hot saturday, a lot of people are most likely going to be there.
It all depends on how long of lines you want to wait in. Personally, I'd go for it. It's an excellent addition if it's going to be a busy day.