Quick thoughts from someone who was able to do both rides via fastpass.
Na'vi River Journey
I may be in the minority, but I was disappointed. Like the movie, the ride was visually stunning, but empty. You really do feel like you are on a boat in the middle of Pandora, but the ride feels like its missing something. There's no story that is typical of pretty much every slow moving ride in Disney. It felt like they spent all their resources on making something visually spectacular and forgot that the ride needed some sort of substance. The thrill rides can get away with a lack of story, but the slow moving ones need to be more than just pretty.
Side note: poor planning on the boat sizes. I know there are going to be some opening weekend bugs, but trying to fit 4 adults in one row was an absolute disaster. We were paired with a couple that were on the larger size and once they sat down my gf followed. There wasn't even enough room left for a small child. She had to get out and we waited for the next boat. Seems like 3 adults is the max per row.
Overall, it's a solid ride but for those who are going during the mass hysteria over Pandora, if you can only do one, do FoP. Personally, I would have been disappointed if I had waited in line 3+ hours for NRJ.
Flight of Passage
The easiest way to describe this is WOW. If you like Soarin, you will absolutely love this ride. Everything feels so real. The wind and surf in your face, the breathing of the banshee between your legs, everything. They left no stone unturned with this ride. I agree with others that its best not to do this ride after eating as you are a bit snug in the seat with the restraints and you really do feel like you are flying and that could cause some post ride nausea if you are full of food. I know some have had issues with the restraints being too tight, neither I nor my gf who is 4'11" had an issue with the restraints.
Pandora itself
It was hard to really get a grasp of everything because there were so many people, but it really it visually stunning. Disney hit a home run. My only complaint and maybe I felt this way because of how many people were there, but Pandora felt on the small side. As beautiful as it is, I wanted more. Felt like they left a little on the table.
The food.
We had the burger buns. They were delicious. My only complaint was that they went a bit over the top on the "earth" food theme. Sorry, I don't want vegetable chips with my burger buns. I want the option of potato chips. Vegetable chips are a cool novelty that goes with the theme, but they tasted bland and I saw a lot of people leave them virtually untouched on their plate. Groups with picky eaters are pretty limited in what they will eat in Pandora. Imo its a simple case of going to far with the theme and considering how many of the Pandora drinks have received poor reviews, I wouldn't be surprised by a pretty substantial menu overhaul in the next few months.
The store
We didn't make it in the store because the line was 2+ hours long, but the banshee puppet people were wearing on their shoulder looked really cool.
Overall, Avatar is really well done. Disney did a great job. There is room for improvement, but they should be proud of what they created.
Advice for those going over the next couple months without a fastpass. The lines for the rides are going to be 2+ hours all day every day throughout the summer.
What really got people was the line to get into pandora itself. It was 95% in the sun and people were waiting 3+ hours just to get in. W
However, I did notice that on both saturday and sunday, there was a lull in the entry line around 3:30 PM that lasted until about 5:30. That seemed to be the sweet spot where the people who had been in line since 9:00 am were leaving in mass and the people who were trying to get in to see Pandora at night had yet to arrive. During that 2 hour period, the line to get in averaged from 45 minutes to 5 minutes. Best of luck to anyone going in the couple weeks for the first time and enjoy.