We are spoiled and will not do DL without MP now. As @Hoopsters says, it really is a game changer. It is very reliable with wifi, however the app will suck your battery dry! Make sure you bring a portable charger. If you are having any difficulties, go to Guest services and they will get you squared away. Once in a while the app can be a little glitchy, but a uninstall/re-install usually takes care of it. In the nearly 2 years I have used it, I have only had to that once or twice. I recommend that everyone go on YouTube before they go and watch a video tutorial. It makes it a bit more understandable.
My usual strategy is to enter the park, step to the side and purchase MP (I don’t have to do that now, as I added to my AP) and select our first FP. Then we do our hour of EE/EMH (we usually stay on site) and then head over to our first FP once the lines start to build. Once we scan into our first FP, I select our next one and so on… If you get a FP more than 90 minutes out, you can select another FP at 90 minutes (sometimes sooner, so make sure to take note of when you can get your next fp and maybe even set a reminder or take a screenshot of when you can make your next fp).
Also in addition to being able to make a FP where ever you are in the parks, you can also make Fps from your hotel room or where ever, as long as you have already scanned into the parks.
Good luck and hope you have a blast!