It's basically just fanservice.
There's a lot more to say than that, but it sums up my overall position on Accel World: Infinite Burst.
The movie begins with an approximately 40 minute long recap of (mostly) the first half of the TV series. This is something I'm not a big fan of, but have seen before in anime movies tied into a TV series and I guess I can understand the intention if it's to bring the audience up to speed with the plot. I still don't agree with that choice, but I can understand it. But here it's immediately ruined by the fact that not only does the recap mostly overlook the second half of the show, but also a long series of other events only covered in the original light novels, leaving the viewer very confused as to what's happening at the start of the animation-new material in the movie if the show is all they're familiar with.
SPOILER WARNING: major plot details follow
These include, but may not be limited to, new characters, Sky Raker's legs having been restored, the fact that Nega Nebulus and Prominence are now one legion (???), the ability of Scarlet Rain's fortress to transform into a four-legged tank and some kind of large flatbed truck-tank thing, and the fact that it seems to have a time limit now, the fact that many characters who were previously only acquainted through the game seem to now know each other in real life and make no big deal of it (despite the fact that the importance of hiding your real life identity was stressed very heavily by Black Lotus in the TV series), and the fact that Silver Crow now throws around Incarnate abilities against AI opponents who aren't using them despite the fact that this was said to be extremely dangerous for one's mental state. I assume most or all of these are explained in the intervening light novels, but there's no explanation to be found within Infinite Burst itself (nor, on a more meta level, for why this arc was chosen to be made into a movie now when so much of its foundational material is unexplained).
The actual plot of the movie is also rather poorly explained and rushed. The hook is solid,that a girl in the middle of a gymnastics accident accelerated into the infinite neutral field right before falling into a coma and has thus been stuck there. We learn that something's going on to disable security cameras and lock people out of parts of the accelerated world, the party gathers at a point outside of the disabled area and is able to pierce the veil using Rain's truck and the Green King's shield, and then they fight a large horde of monsters summoned by a legend-class enemy Nyx, who has apparently been summoned by the trapped gymnast, and Crow and Lotus enter her fortress and have a brief confrontation. This all has the makings of an interesting narrative with some good tension and reveals, but it goes by so fast that nothing has much weight; the new content of the movie only makes up about half of its 80-minute runtime, only about 1.5 times the length of a single episode of the TV show. I have no idea why half of the movie was given to recap when the other half feels so rushed, but regardless of why, it severely hampers the story.
That being said, the art and soundtrack are still great, the characters are likable for what we see of them, and some of the fights are quite well animated. Seeing this stuff in action is cool for people who already know the story, or fans of the original show that want to see more stuff from the characters they like, but as a movie it's pretty weak, and I genuinely don't understand how the same people that made the TV series, which I think has great pacing and narrative construction, could have messed up this badly on the movie. I don't really know enough about how anime is produced to speculate, and I'd love to hear an explanation if anyone has one. Overall, I wasn't expecting anything as great as I thought the show was, and I was still pretty disappointed.