I booked two sets of fastpasses last week, one for my trip and one for my friend's trip. I usually do them on the computer, but my laptop was having issues the morning I had to book for my friend so I used the app. I was able to use my laptop for my fastpasses a couple days later, and definitely prefer doing it that way. I really didn't like doing it on the app, but it depends on what you're more comfortable with.