The beginning and exposition are a little slow, granted. But it is worth it, it allows you to enter peacefully in the story.
And as it goes on, the story gets more original as we see the monsters are not necessarily who we think.
I did not expect it to be a musical, too, and really, that's not it. But the story revolves around a singer, so she sings (and I stop here so as not to spoil). In french, she is voiced by Vanessa Paradis (yes, Jonnhy Depp's Vanessa) who is a pretty good singer and actress. The songs are pretty good, the other singer (pretty famous in France, too) add some poetry to the thing, but the music is not too much. There is no singing for no reason like in the Disney cartoons.
The rendering of Paris, during the great flood of 1910 (true story) in very well done and the atmosphere is beautiful.
There are a few situation jokes ; you don't explode with laughter but it's funny and you can see that the writers prepared them carefully (the thief joke, for instance). Plus, in the french version, there a plenty of little allusions and puns that are really fun.
All in all, a very enjoyable experience for kids or adults.