The short and skinny, don't watch this show, and if your kids like this show, go and buy a DVD of either Dexter's Laboratory or Jimmy Neutron and show it to them that there's better programing because you don't want to set your children's standards low at such a young age. There is nothing like having your child become 'that kid' who ends up making the same paper mache volcano every science fair.
Now for the nitty gritty. Johnny Test is a pale, cheaply made imitation of "Jimmy Neutron" from Nick. Heck, just compare the main character's for starters. Johnny has tall wild hair, Jimmy has tall wild hair. Johnny is short statured and bullied by the dim witted gruff bully, Jimmy is short statured and bullied by the dim witted gruff bully. I can go on but I only have a 1,000 word maximum to write this review.
Character wise though, Johnny is not entertaining at all, kind of playing the 'lowest common denominator' of characters. Basically a brat who begs his sisters to make him cool stuff that he ruins and points out the obvious and then at his sisters so that they can clean up Johnny's mess. Jimmy on the other hand was the genius and was made fun of it. He also messed up but that's because of trial and error or maybe something someone 'else' did. Not to mention he cleaned up his own mess, and in a fun and unique way too, showing what he was going to use and for a few seconds you would sit there puzzled wondering how he was going to Mac Guiver it together.
The animation is appalling, basically Adobe Flash drivel. In one episode I even saw a part of a character vanish because it was the same color as the background. Flash isn't 'bad' (I suggest you watch "My Little Poney: Friendship Is Magic" to see some true Flash artwork), it's just that it was made lazily.
Story wise, it's pretty much the same every single episode, with no deviation what so ever. Johnny gets in trouble/wants something/is too lazy to do something, begs his sister's, abuse their invention, states the obvious, cries to his sisters to fix it, sister's fix it and Johnny is declared a hero with something funny at the end. Wash, rinse, and repeat. Though, I must admit there has been a funny moment here and there, it's overall boring and predictable to the point that new episodes are hard to distinguish from the old episodes.
I suggest that you stay away from this horrific show and produce some quality television.
