I managed to find a fastpass for the River ride and got on within 5 minutes. I would not wait more than 15 minutes to ride this, but I will give my thoughts on the ride without focusing on the wait time. Surprisingly it seems I will have a somewhat different opinion than some here.
I thought it was VERY good. Almost great, so close to being the ride I ideally wanted out of this land but not quite reaching it. It provides something the park needs desperately, a calm ride that anyone can enjoy including those with issues preventing thrill rides. And full of lovely eye candy, gorgeous lighting and little details everywhere. The effects mostly aren't even cutting edge, they are just extremely well executed, well designed and there is a lot of them.
And i'm glad the scenes are so densely themed. One big issue i've had with many recent rides are the inconsistent or sparce theming in them, or lots of visible backstage stuff that breaks the immersion. Little Mermaid for instance has a lot of the structural elements like lighting rigs that are very visible. It doesn't help that most of the show scenes are also poorly designed with mediocre sets and figures, and poor lighting. 7DMT has its excellent Mine Scene (and a nice cottage scene), but the exterior scenery is nowhere near as detailed and suffers especially at night with very poor lighting. Navi River is at least an infinitely better ride than Little Mermaid IMO, regardless of what alphabetical ticket rank Disney internally classifies these as. And at the very least more consistently themed than Mine Train (though yes the Mine scene is excellent and has some nice and well designed animatronics). I would also bring up Expedition Everest's backwards tunnel, which is unthemed and exposed to the naked and very visible steel interior. Even Dinosaur (while the scenes it has are well designed) has a large chunk of the attraction occur in complete darkness due to intended scenes having been cut. Frozen has similar lapses in show quality too.
So one phrase I would use to describe the visual element of Navi River- consistently well themed. Look around all you like, there are no lapses in quality or "dead zones" where scenes are more sparsely detailed. It's the most consistently well themed WDW ride in a VERY long time, perhaps since the Safari. The imagineers did an excellent job of not only designing the scenes in the ride, but also clearly made the effort of thoroughly hiding unthemed backstage elements.
So is there a flaw? Absolutely. It should have been AT LEAST 8-10 minutes (which according to Martin WAS the original intention). I consider myself lucky that the ride actually stopped for a couple of minutes and artificially stretched it out. The ride also needed more animatronics. Not all have to be of the Shaman's movement (which is excellent btw), but they still should have had some others. Though more importantly the length is lacking, it's over far too soon. That being said, I am glad for a change to have a ride being good enough that I wanted to see more of that, as opposed to my criticism being able to point out a huge list of poorly designed scenery elements, figures, lighting etc.
This is not a thrill ride, and i'm glad of that. While I wouldn't have said no to having a drop as the original form was apparently intended to have, the lack of one isn't something I would count against it either. It's my idea of a relaxing "zen" type experience with just a lot of lovely eye candy. The length is a buzzkill (admittedly a serious one that prevents the ride from reaching its full potential), but i'm still a big fan of Navi River. Mom loved it too (same complaint about shortness though). I haven't been on the Flight sim yet and don't expect it to be my cup of tea. Thrills don't bother me but simulators don't really interest me.
People will probably be pretty irritated if they wait an hour or more for this though. Fair warning.