Considering that this book was published in 1872, I was surprised to find that it made for fairly quick reading. This is helped significantly by the fact that the author addresses the reader directly at the start of the story, helping to ease the reader out of the real world and into the fantasy world he has created. (I am someone who usually has a hard time understanding any world which does not directly mirror the real one, so this sort of slow immersion in the princess's world was very helpful for me.) The descriptions, though somewhat old-fashioned in tone, paint beautiful pictures of the author's vision of Irene herself, her nursery, the mines where Curdie's family works, Irene's grandmother, and Curdie's own parents. It is also easy to conjure up grotesque images of what the goblins must look like, which adds just enough of a sense of fear to the reading experience to be fun without causing nightmares.
There is a ton of Christian imagery in this book, and though I knew to look for it, many of the references are so subtle, that I know I didn't grasp their full meaning on this first read-through. The theme of believing without seeing is the most overtly religious message in the book, but there are also hints at the importance of love, forgiveness, and keeping one's word that certainly support a Christian worldview. I don't think I enjoyed this much as I have enjoyed the Narnia or Lord of the Rings books, but I could certainly how it would have influenced Lewis and Tolkien in their own work, and I definitely think readers who enjoy Lewis and Tolkien will also love MacDonald.
In terms of sharing this book with kids, I would say it is appropriate for kids as young as four or five to hear as a read-aloud, but I would caution against having this be their only experience with the book. This is a story that has many layers and can be appreciated differently at different developmental levels.
I have some unanswered questions left after finishing this book, and I suspect it will probably be necessary for me to read the sequel, The Princess and Curdie, in order to find out their definitive answers.