A wonderful story about creativity and facing up to fear, beautifully animated and with a lovely soundtrack. The filmmakers used a wide range of sources for inspiration, everything from Celtic illumination itself to Soviet-era propaganda posters. It's all melded together into a style that isn't flat at all, even though it's not slick or 3D or CG or anything.
Historically, it's kind of naive (intentionally so, I imagine). And some of my Northern Tradition friends won't like the way the Vikings are handled... basically as faceless villains.* On the other hand, they're really not the point of the whole story... I don't think it (or any other of the broad caricatures in the film) detracts from the beauty and the power and the message. And after all, the monasteries of Iona and Kells both were sacked many times by the Vikings...
The trailers are good, but also watch this:
* with horned helmets, mohnkern! ;-)