Last month we tried using the assistive listening device at WDW. My DD7 uses an FM system at school but does not have regular hearing aids, and we were hoping the device would be similar (using a headset). However, we found that it had very poor reception, when it did transmit the audio through the device didn't always match the audio I heard in the attraction (and not just a delay of seconds - think Carousel of Progress and hearing and entirely different section of the show). Often by the time we got reception through it, the show would be half over, or it was such spotty reception that DD eventually said she didn't want to use it.
I don't want to discourage you from trying the device - hopefully your experience will be better than ours. Just wanted to let you know that we didn't have good luck with it. I might try it again another time, especially since it's essentially "free" (they charge your CC but credit back once you return the device).
I should add that our DD's reading ability isn't such that we are ready to try to captioning device, so I can't speak to that.
Good luck!