This is a kids' movie. There are numerous jokes involving bodily functions (both human and dog), some cartoon violence that includes incidents which could kill a person in real life, and one scene where Monica is showing too much of her chest. But it's appropriate family viewing. It would have to be because no one but kids under 12 could possibly be that impressed with it.
Joey Diaz is a great moron who gives the standout performance as Stewey.
Dean Cain has done better in his career but he's entertaining enough.
Lance Henriksen gives a quality performance (for this material), at least in his first scenes with George, just creepy enough but not truly scary, so there's no concern for the kids.
Elisa Donovan is the voice of reason and out of the remaining actors playing humans comes the closest to giving a good performance.
Ariana George as Shelley the Spider, Eli's pet, gives us an interesting performance. Only other animals can hear her. Mayim Bialik as Medusa is also pretty good, more angry Blossom than shy scientist Amy.
Zeus is well trained, but his voice actor is nothing to write home about, except in one memorable scene where the dog tries on Halloween costumes.
Mindy Sterling (as Grandma) is not Lily Tomlin. There is a resemblance but Tomlin is better.
The visual effects are effective but not too scary for kids. Eli's house is appropriately scary with moving suits of armor, paintings with eyes that move, and the other kids' horror movie stereotypes.
As for the solution to the big mystery, and the heroic deed of Zeus: only kids will be impressed. The only reason for the movie's name is to connect it with what appear to be numerous sequels to the first one. I can hardly wait.
Oh, well, I had a good time.