If you like Neil Gaiman's writing, go see it.
If you like Dave McKean's art, go see it.
Heck, just go see it if you have the chance and find the trailer intrigues you even in the slightest. (Though the trailer didn't do the film justice.)
Mirrormask is getting unfortunately very short runs in very limited release (one week, one theater per city), and is getting some unfortunate reviews from some 'professional' reviewers. I can't understand this.
The audience in the theater I saw it in with shadowfey tonight were thrilled by it. The projectionist showing it -- the Seattle run opened tonight -- got so caught up in it that they *forgot to change the reel over halfway through*. The audience joked, during the pause, that they were going to come and tell us 'okay, now, for the second half of the film, you have to pay $30 for the other ticket.'
We pretty much all would've paid it.
This film instilled that same sort of sense of wonder that many of us had from The Dark Crystal and Labyrinth (and The Neverending Story, as well)... which I don't think most of us had gotten from a film in a very, very long time. There were bits of dialogue so perfectly timed that the audience howled with laughter, or applauded the characters. When the movie ended, there was a round of thunderous applause, and much talk among people about coming back to see it again during this little one-week run.
It was creepy, and surreal, and funny, and even a little thought-provoking. And definitely worth the money, at least in my opinion. And evidently the opinion of most others who've seen it and posted to IMDB or other movie-related forums, having driven two hours to get to the one theater showing it and so on.
I won't say more or really go into the plot, because I don't want to spoil things for those who also want to go see it.