Haven't been following this thread because I like to ride spoiler-free.
It's fun! It's good. It's the most fun ride at Epcot, but that isn't saying much. It is not incredible. I would like for the people calling it the best ride at WDW to take a step back and think about the fact that your first ride on this is obviously going to be more exciting than your 20th ride on Flight of Passage or (insert ride here). The queue and buildup are quite good and might have been ranked as the best at WDW, were they not already previously surpassed by Flight of Passage and Rise of the Resistance (okay, Rise's actual queue before the first pre-show isn't great but everything from the pre-show onward tops this).
I probably don't have much else to say that hasn't already been said.
I was slightly underwhelmed by the thrill level of the coaster. I was hoping for Hagrid's level of thrills, but it's not there. In my opinion, Everest remains the most intense WDW coaster, with Rock 'n' Roller Coaster coming in second for its launch alone.
After such an elaborate queue experience, the coaster portion, devoid of ANY physical sets, props, or animatronics, feels a bit empty and very unbalanced. One brief slowdown at a physical set with some animatronics halfway through would have gone a long way. I agree with everyone who said the brightness of the screens kills the illusion in many areas and seems like something that should have been considered for such a massive price tag attraction. I also didn't like that all of the "story" during the coaster portion is conveyed through onboard banter dialogue. You don't even see the Guardians, you only hear them. You feel disconnected. I also found the dialogue volume to be too low and a bit muffled. There's a big issue with this because the ride, being a coaster, means people are screaming and also singing along, making the dialogue even harder to hear. This is something I hope they work on.
I was under the impression that during the ride we would be rewinding through time while trying to stop the bad guy, and honestly that is a way cooler concept than "uh oh, you just got here but we've gotta get you outta here!", which felt uninspired, so that was also a letdown.
Finally, does this fit Epcot? No. If Xandar were a real place and they were showcasing real technology, then yeah, sure. But, since it's all fictional, it's a bit of a cop-out. The ride portion being only an escape from a bad guy and nothing to do with the initial premise only further muddies the forced "connection" to Epcot. However, it does fit better than Frozen, Ratatouille, and Nemo, so that's something.