Yes everytime we've been there it has made a huge difference, at least the times when we went during peak times. Now I have been in off peak times and it doesn't really matter because the lines are small anyway. But yes during peak times, it has for the most part, not literally got you to the front of the line, but reduced say an hour wait to 5 minutes or less, typically. At least that has been my experience. Now this will be different with the HP area because I think they don't allow the FOTL passes for the new HP ride, but this should be the only area that is effected that I know of. But from what I've heard you probably don't want to do FOTL with the Forbidden Journey anyway, because there is so much in the line to experience and view that it is like part of the experience anyway.
This is also the case for lots of the rides. If you have time I would suggest not using FOTL passes to experience the long line. At least for some of them, like MIB and the Mummy, there are really cool things in those ques that are not to be missed, that you will miss if you use the FOTL pass, but it all depends on how much time you have. For example with the MIB que, in the long line, you pass by the coffee break room where the worms are and they are talking and saying funny stuff, not to be missed, but you skip this part if you go through the FOTL que.