It's pretty off the beaten track, but Crunchyroll streamed it, so I watched it.
It's a recording of a 10th anniversary live stage show for Sword Art Online. The anime voice actors appear as themselves and read scenes (holding scripts in front of them), making no attempt to resemble or even dress up as their characters. (Lots of asymmetrical outfits for the actresses.) Miyuki Sawashiro is probably the best fit for her role and I'd cast her as a live-action Sinon in a heartbeat, but the men are all slightly disconcerting. Yoshitsugu Matsuoka is too fat for Kirito, while Hirata Hiroaki is sixty years old but fantastic as the cheerful young idiot Klein. I found it fascinating to watch him.
Partly just to confirm who's playing who, the characters' pictures are projected behind the actors on gigantic screens. The scenes are a bit variable. There's no plot. It's just a bunch of one-off conversations from throughout the franchise, from the original Aincrad arc to the latest Progressive films (including Mito). Most of them are nice enough, but vaguely pointless. The first one is unfortunately also the worst, with Asuna wondering how she can sneak past her guard when leaving her prestigious apartment. (Is that necessary? You're the leader of the whoever-they-are. Just stroll past and say "I'm popping outside".)
Some of them, though, involve dead people. SPOILER 1 pretty much made me cry and SPOILER 2 ended up pretty emotional too. (And he's a more convincing romantic partner for Kirito than Asuna.) You'd have to have watched the series already to know their fates, of course. The stage show assumes you know everything already.
I also discovered that I love that little AI fairy, Yui. Her conversation with Alice is also meatier than most.
The music's a big deal. Lots of singers, with lots of music from the different films and TV series. This narrative-free material couldn't have supported a three-hour stage runtime without any musical numbers. There's a good hour of that all told and I ended up fast-forwarding through quite a lot of it (good though it is)... but not all. I listened to all the incidental music they played around the halfway point, which I enjoyed a good deal and I wouldn't call them songs even though they use opera-like choral singing as an additional instrument.
The most jaw-dropping bit was when an anime character materialised on stage and started singing. How did they do that? A hologram? A laser projection? Obviously computer-generated, but how was she projected? However they did it, it's brilliant... especially since the image quality's perfect even when smoke's bellowing up on either side of her. She even jumps on a flying saucer and zooms out into the auditorium, above the audience's heads.
Oh, and the stage show also skips through all the Sword Art Online timeline. I suggest trying to keep an eye on what year/floor we're currently on.
Is it good? Well, it's amazing. It's worth putting on in the background, but don't expect a narrative. It's just three hours of stuff that people thought might look cool as a stage show. There's some good music and I found it interesting to see the voice actors stepping out in public to perform. It's worth putting on in the background, at least, although the scenes will mean nothing unless you've seen Sword Art Online.