I'd get one FP so that you have a chance to ride it without a wait.
Frankly, though, I am absolutely shocked at how SHORT the standby line is for a brand new ride (as in technically it isn't even supposed to officially open until tomorrow, lol). They never really did much of a soft opening, the opened it for the 24 hour day last Friday and have been offering FP+ since.....we did our FP+ at 7:20 tonight and the standby was 75 minutes posted. My girls decided they wanted to ride again, so I went and found a comfy seat at Village Haus thinking I had 75 minutes at least......they were done with the ride and sitting down at the table with me in 55 minutes. Sure a FP is better than an almost hour long wait, but this is opening week for this ride and that's as long as the wait is? By the time you come it may be half that long.
It was an okay ride......a much slower moving train than BTMR and while it has a few fun twists and turns, still not comparable to BTMR for thrills. Now....my opinion, that makes 7DMT the perfect alternative for those with kids who aren't quite ready for the speed and thrill of BTMR.
I must say I was disappointed at how cheesy the "jewel mine" scene was....big honking plastic "jewels" stuck randomly to the wall and the Dwarfs swinging picks, etc....Dopey with two big jewels stuck in his eyes. Looked like the jewels you can buy at the dollar store, only bigger, but same cheap look. It all seemed very tacky and not at all Disney's usual classy scenarios.
The scene of Snow dancing with Dopey was cute, but again, the theme inside the cabin just didn't seem to have the detail that I expected, and the animatronics seemed very stilted.
Not sure if I was expecting too much, or if Disney truly failed to deliver their usual quality. Would be interested in others opinion on this point.