It's more Sword Art Online, which is a franchise I like without being a raving fan of it. It's pretty good. I enjoyed it. That said, though, it's basically more of the same and the things it does differently are minor tweaks that non-fans probably wouldn't notice. If asked to describe it, I'd probably just say "it's more Sword Art Online". My anime-watching work colleague likes this franchise a good deal, but even he couldn't remember whether or not he'd watched this film. (He had.) It's another slightly unmemorable standalone movie that's been sandwiched between two seasons of a TV series, if you're going to be brutal about it.
That said, though, it's clearly better than the latest movie for something like One Piece, Doraemon or Lupin III. This isn't yet another never-ending shounen franchise. I've seen this on "best anime film of the year" lists from intelligent reviewers and I wouldn't argue with them.
This isn't the franchise's first film, but it's the first one that's not just a compilation of TV series episodes. It's also based on an original story, instead of adapting one of the light novels, and it's set between seasons 2 and 3 of the anime. It has almost everything from the series, but one thing it doesn't have is non-stop wall-to-wall action. Instead, it's capable of feeling oddly mundane. The fights are spectacularly animated, but they're not what's driving the film's plot and so they're capable of feeling almost peripheral. The story's set in the real world, with most of the fights just being game events that no one's being forcing into and so the stakes might not feel particularly high. If anything, rather than being an action film, you could almost call this a detective story.
The story's built around VR (Virtual Reality) vs. AR (Augmented Reality). The former is what everyone had been stuck in for two years in Season 1, with game-death being fatal in real life too. You put on a headset and the computer creates everything. Your real self just lies on a bed, or at least that's how it works in this show. Augmented Reality, on the other hand, means wearing spectacles that make you see virtual things in the real world. You're supposed to do it while you're walking around and exploring real environments.
In this film, AR's the latest big thing. Ordinal Scale is a company that lets you fight monsters in Tokyo and even has its own computer-generated AI idol as well as an AR gaming system. Kirito's being a bit of a wet blanket, saying he prefers full-dive VR, but that might be because AR involves moving your real body. Kirito's a teenage nerd who doesn't work out or even eat properly, on top of having once spent two years bedridden. You remember how he used to be Captain Unearned Awesome in the TV episodes? Well, that's less true here.
He's also got a girlfriend, Asuna, who's equally strong at VR games but seemingly a good deal better than him at AR.
BETTER IN THE TV SERIES
Most of this film doesn't feel very urgent, to be honest. I enjoyed it, but it's nowhere near the "fight or die" intensity of the first story arc.
I don't know if I believed in the memory technology. I suppose one has to accept it because this is Sword Art Online, but even so I wasn't convinced. How does it work? Can you even do that? Surely you can't just suck out memories, whereas scanning I could have believed in. (Answer: had it been just a harmless scan, there wouldn't have been a film.) There's the question of whether/how that kind of thing could/should be reversible. Hmmm.
BETTER IN THIS FILM
Kirito isn't Mr Supreme-balls, instead at first being rubbish at this new kind of gaming. (He'll get the hang of it eventually, but even then for once he can't beat the final boss by himself and instead we get a massive final callback.) Asuna's a better fit in many ways for the film's hero role, getting the juiciest fights and then later the strongest emotional material. She shines as both a fighter and a leader. This is a good film for Asuna fans. Kirito's the one who solves the plot and does all the detective work, though.
It's also nice to see development for the Kirito-Asuna romance.
It's nice just to be reunited with everyone. The film's a direct continuation of the original Sword Art Online story arc, unlike all the other sequels, which makes it a pleasant surprise when, say, Sinon shows up. Seeing all those old faces in the last battle was a bit emotional, even for me... and I don't even remember the earlier TV series that clearly! It's been a few years now. The film's most fleeting cameo is also its most powerful one, incidentally.
The art and animation are top-quality, whether you're here for the big boss fight scenes or a glimpse of Asuna in the bath.
The film's actually about something. The baddies have pain and motivation. "Those of us who couldn't fight aren't worth remembering." "Weaklings like me and Yuuna are always overlooked." There's the question of whether survivors might welcome the loss of their bad memories, while certain scenes are inviting tragic real-world parallels with dementia, Alzheimer's, etc.
Various minor fan grumbles about the series get subverted, e.g. Kirito doesn't wear black all the time, the new girl doesn't fall for him and you'd never guess that this is a franchise that's been accused of verging on harem at times. There's also no reminder that Kirito's "sister" ever had inappropriate feelings for him, although that's partly because she's being sidelined for the sake of plot balance. (As one of the country's top four kendo players in her age group, letting her loose in an AR fighting game would have been like releasing a shark in a goldfish tank.)
NOT NECESSARILY GOOD OR BAD
Kirito has some slightly negative traits that mean he's not just another bland male lead. He lies a bit too easily, more than once, and he does something both wrong and stupid in reading Asuna's diary without her permission when she might return to the room at any time. (Fortunately she's okay with it, but look at Kirito's brief guilty reaction.)
It's got everything you'd hope to see from this franchise. It's got all your favourite characters (and I do mean all) in a story built around the latest game technology, with cool battles and even some progress with the Kirito-Asuna romance. It's pretty much what I'd expected, to be honest, but in a good way.