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Just finished this DLC. It's very solid, especially if you enjoyed the digressions and battle recollections that let you play as other characters. You can play as any of the 7 Servants actively participating in the Waxing Moon Ritual (except for Caster's story, his is different). The overall story structure is the same for each character (to the point where a lot of scenes later on in the story are 90% repeated on Gilgamesh and Ibuki-douji's ends in almost every route), which can get repetitive if you do them back-to-back like I did, but you still get some nice characterization for the various Master/Servant teams in each story as you play through them. Ibuki-douji's characterization is fun enough that I didn't mind seeing her do her thing several times over, but I'm also already a preexisting fan of her from FGO.
You start out with access to the stories for Saber, Rider, and Berserker. Beating each of those unlocks a specific other Servant's story. (e.g. if you finish Saber's story, that will unlock Archer's story. Finishing Rider's story unlock's Lancer's story, etc.) Caster's story is either unlocked by finishing two stories or by finishing a story that's unlocked by finishing another story (I unlocked Caster's story by finishing Archer's story, so I'm not sure which). Assassin's story requires you to beat Berserker's story and be a bit of the way into chapter 2 (I did it after getting to the "investigate Assassin and his Master" quest, but it might be unlocked as soon as you leave Zheng Chenggong's place at the beginning of the chapter) - this is only really an issue if you're starting a new save file to play this DLC after beating the game, like I ended up doing, otherwise most files will already be at least that far in.
An additional note for people looking to play this DLC: there's a subquest to place 7 golden statues, which unlocks a new bonus boss. More specifically, placing the 7th statue immediately puts you into that boss fight, so be careful who you want to complete that subquest with (I ended up finishing it with Assassin, whose story I unlocked last, and having to do the fight as him despite him being my least favourite Servant in terms of move-set). Mechanically, playing it as Berserker might be an interesting choice. Caster would probably be my recommendation, partially because of how unusual the Servant you control is, and partially because the boss had specific commentary for Assassin (which means the boss probably has specific commentary for everyone, and I think Caster's would probably be the most interesting).
Outside of the self-contained DLC story, this DLC also unlocks the ability to play as Rider during Battle Recollections, as well as to have the Boss and Ibuki-douji accompany you as Rogue Servant options (each is a Rogue Ruler Servant, and Ibuki gets the Ruler class glyph shaped skill tree). The Boss has no skill tree and no Noble Phantasm, but Gate of Babylon is mapped as his only Affinity Technique, and it's as OP as you'd expect. Both are also playable in Battle Recollections, if you want to use them in any of those. Finally, Saber and Iori get a new Affinity Technique (purchaseable in the DLC-specific shop for in-game currency), and unlocking the Boss as a Rogue Servant gives Iori a new spell (one that mimics Gate of Babylon). The DLC also provides workshop upgrades (materials purchaseable using the same in-game DLC currency) that extends (and then further extends) the amount of time a substituted Servant is on the field, for even more fun playing as Ibuki-douji or Gilgamesh (or any other Rogue Servant you enjoyed playing as).
In conclusion: Ibuki is great, the Boss is great, the characterization is fun, the story is a nice wacky diversion (that secretly has something more serious going on, as you should expect from Fate by this point). The structure is pretty samey (especially playing through it with 7 different characters/teams), but not enough to get tiresome. Certainly nothing compared to doing an entire extra playthrough (or reloading an old save from chapters ago) to get a slightly different version of the third ending to the main game for that Complete Logbook achievement.
Strongly recommended if you enjoyed the other Master/Servant teams, if you're a fan of Gilgamesh or Ibuki-douji, or if you really liked the Rogue Servant concept and want even more friends to call upon (watching Ibuki fight battles in the main story is very entertaining). Archer even gets to have a happy ending in his story, unlike in every single route for the Waxing Moon Ritual.