It's a ballet-themed magical girl show where the heroine is a duck. However it's also an important work that's attracted some heavyweight praise and is extremely well-regarded. The premise and title might put off some viewers, but that's their loss.
On actually watching it, though, my reactions were complicated.
It's dealing with fairy tales and storytelling. Every episode begins with a narrator telling us a short, grim tale in fairy tale fashion. We're watching events that are pre-ordained to end in death and despair, because that's what storytellers are drawn to. (Well, this one is, anyway. He's a creepy old bastard.) Tragedy is cool. Happy endings are boring. Meanwhile the story's set in an ostensibly normal middle-European town, except that the protagonist's a duck and the town's full of animals who talk like people and no one even seems to have noticed. The school's ballet teacher is a cat. There's a giant talking anteater girl. Sometimes the fantasy goes beyond that, though, into puppets. Random characters in the story will be dolls. They'll look and move like puppets, but they'll talk to you and run tea shops.
Then there's the ballet, complete with a classical music soundtrack to die for. (Lots of real ballet music.) I wouldn't call myself a dance fan, but I worship whoever thought up the idea of building this anime around ballet. Ahiru/Duck is a magical girl whose idea of battling her enemies is to do some ballet dancing. How often do you see that?
It's beautiful and lyrical. It's intoxicatingly stylish... but most of the show's episodes don't actually have much underneath that. It's still basically a magical girl show. Ahiru has to defeat baddies and collect magic shards. There's not much dramatic momentum. Everything about the show is enchanting and unique, but fundamentally, for the most part, it's not really going anywhere.
That was my reaction. I wanted to love the show, but episode after episode would fail to move me. They'd always be pretty and eccentric, though.
Then, though, we reached the finale and everything changed. What I'd thought was style turns out to be literally true, on levels I hadn't expected. It's an ending that changes everything. I suspect that rewatching this show will be a completely different experience, as your knowledge of what's going on lets you see Ahiru's world through new eyes. One day I'll rewatch this. I'm even looking forward to it. It's an ending so strong that it raised my estimation of the entire series, making me forgot how ambivalent I'd been about much of it. It had never been bad, or anything even remotely approaching it, but... well.
Imagine a music box. It's an antique, probably Victorian. It's got intricate metalwork that looks like lace. If you wind it up with a tiny key, then it'll play delicate, slightly haunting music for a few seconds. That box is like this show.
The cast are archetypes, but the story's also about how they'll struggle against their story roles. Ahiru is a duck who can turn into a schoolgirl who can turn into the ballet-dancing heart-collecting magical girl, Princess Tutu. There's a prince, Mythos. There's a bad boy, Fakir. There's a dark magical girl, Ruu, who also claims to love Mythos and is... well, she's more grown-up than Ahiru. (Ruu's sexy and gets eyebrow-raising outfits, whereas Ahiru's duck-human transformations always leave her naked and yet are never played for anything but laughs.)
The show can be good fun, by the way. It's not just an intellectual experience. It's still a children's show for girls, after all, despite having been filtered through some deeply strange Japanese brains. Ahiru's school friends are a laugh, with Lilie being everything you could want from a girlie, cutesie best friend while also being a black-hearted sadist. Cat-teacher's favourite threat is to make his underage students marry him... but he's a cat. He's charmingly mad (and also voiced by Yasunori Matsumoto, Tomoko's favourite voice actor from this cast). Then there's the one-off guest star in ep.17, the utterly mental "true prince" who loudly mourns his own sinful perfection while his servants blow clouds of rose petals past him.
It's complicated. It's a fluffy pastel-coloured magical girl show with talking animals and lots of ballet, but it's also a dark mind screw with a complicated ending. The second half is a heartbeat away from being about a serial killer who takes people's hearts. In this world, you chop off storytellers' hands with your axe. I don't think I can process it properly without a rewatch. I can think of other examples of the same fundamental idea, one even in Doctor Who, but none with this aesthetic and none that felt the same by the time I'd reached the end. This show's both complicated and transparently simple. It's special.