Lots of fun for kids, but maybe not as much for the parents. Jim Carrey is thoroughly "on" in this movie as the cruel, somewhat disturbing, but imcompetent-when-it-counts Count Olaf who covets the fortune that the children have inherited. I might go so far as to say he's the star of the film, much in the same way the Joker was the star of the first Batman movie.
The children encounter all sorts hardships and have to use their wits to escape both treachery and forces of nature, but as the eldest of them says: "There's always something." That is, there's always something you can use to your advantage; don't give up, keep looking for a way, there's always something that can help you (even if it's not obvious, even if it's not the "intended" use). So we have a continuing theme of self-reliance and perseverance in the face of impending doom, which is great stuff.
There is another theme that is not as prominent, but still worth mentioning. The children face terrifying animals (snakes and leeches) as well as terrifying forces (hurricanes and heights) but these are not the worst. But the worst things they face are other people: malicious people, indifferent and incompetent people, clever and conniving people. I think the lesson here is that nature can be dealt with, but other people, well, you have to watch out for those. (Compare with my "Rule Number 1".) Family, though ... family is sanctuary.
There is one scene toward the end that really disturbed me: Count Olaf attempts to marry the 14-year old daughter (the eldest). Made my skin crawl while I cringed away from Olaf's leer.
Rating: worth a matinee viewing, but take the kids with you.