I'll gently disagree - I found FP+ at WDW to be very, very useful to me as an off-site guest. But that's also because I knew the tricks.
I knew I could time my three morning FPs to be back-to-back-to-back, arrive at the park near the end of the first FP time, and get three great rides in the span of a little over an hour, then bash the app the rest of the afternoon to see what other juicy FPs I could get.
Sometimes, I would play "FastPass Roulette" at the hotel in the morning - bash the app for ten or fifteen minutes and see which marquee attraction it might spit out. That would tell me which park I would start at that day. (One Saturday morning, I was driving from MCO to Tampa and knew I had about an hour to spend at a park that morning, so I bashed the app for ten minutes. It gave me a Slinky Dog Dash FP for right at the end of that hour. On a Saturday.)
("Bashing the app" kinda worked with MaxPass at DL, too. If you went forwards and backwards on the MP page, it would sometimes give out, for example, an RSR FP for like an hour from now - though you had to be very, very fast to confirm it because the app wouldn't hold onto it for any length of time, unlike the WDW app.)
My favorite trick - they tended to release Flight of Passage FP's every day between 3pm and 3:05pm, so I knew I could just bash the app for those five minutes and have a reasonably high likelihood of getting one. I almost felt guilty walking into FOP at 4:30pm, knowing that I had gotten the FP at 3pm, and skipped ahead of people who had been in line since 1pm.
Of course, that doesn't mean it was great for anybody else. But I got miles and miles out of it.