Just got back yesterday. Spent Thursday at AK and had a FP to Flight of Passage. I then obtained commitment from my wife to wait the 75 minute wait for River Journey.
Land and Immersive Qualities: Like many have stated. The rockwork is fairly amazing but not mind-blowing. Much better at night for us. Did enjoy the flowing water and setting but kind of felt like they missed with it. Our preference would actually have been a beautiful tropical or island setting and not have committed to Avatar. Just us.
River Journey: So, we went to the restroom beforehand and then got in line. Waited forever in one spot, moved then feet and resumed waiting forever. I hadn't really watched the videos of the queue and was shocked that, once we went into the "cave", we were ready to board. It was a long 75 minutes, though. We boarded and moved into the ride. From our perspective, it all felt fake and not immersive. The screens didn't draw us in. Some parts were cool, like the six-legged frogs jumping fronds, but it mostly, "meh". We disembarked with the same, "That's it?" I didn't think that story mattered much until that point. Nothing existed to draw us in emotionally.
Flight of Passage: 2 1/2 hour waits. All day. So, I was happy that I had committed to logging on 64 days ago nice and early and obtaining Fastpasses. Get there for our window and start walking the exterior queue and then into the building through the concrete bunker. Here, my wife and I disagreed. She was really questioning things while I was assuring that the lack of theming was the theme (since it was a military bunker after all). Then, we hit the pre-load shows. Two things. First, not going through the queue and seeing the indoor setting with plants really stole some of the experience. I had watched Youtube videos of the regular queue and looked forward to going inside. Second, the pre-shows are boring and way too long. Oy. As far as the ride, it was very good but not overwhelmingly so. I was on the very last "banshee" of the row at the far end. For some reason, I could see the other riders and also around the edge of the glasses. My wife enjoyed herself, which was good. No way is it worth that wait, though. It was over about a minute too soon. While some may disagree, I think that Spiderman does a much better job at putting you into the ride.
Overall: Nice land but overrated. While I don't know the final cost (some estimated it between $500m - $1.0B), it was a missed opportunity. It definitely needed a third ride, higher capacity rides, and more story line on the River Journey. As we left the park, my wife asked me how long before it becomes less popular. I responded early 2020, once SWL opens, and then hard to say what impact it will have. It just didn't have that staying power that one thinks of with Disney. More like something that could be easily reskinned into something else if they don't want to keep paying for the licence.