I have a hard time believing that I actually used to like this abomination when I was younger. I was blinded into liking it by my interest in Jimmy Neutron, which is much better. If you're a fan of Jimmy Neutron, you probably won't be a fan of this.
Even though this is a spinoff of JN, little to no connection to it is made here, and the latter completely ignores the events of the former. Unlike the former, the plot of this doesn't make sense. Also, the theme song has all sorts of weird and nonsensical sound effects in it, which makes it super-duper annoying. Sheen is the only returning character here. He was originally a nerdy, hyperactive, wacky, dimwitted, and hilarious goofball, but here he is annoying, obnoxious, self-centered, and immature. The writer of Jimmy Neutron has said that Sheen was fun to write, but only in small doses. These circumstances are signs that comic reliefs aren't always meant to be the main protagonist.
Where are all the other Jimmy Neutron characters? Some of the new characters are carbon copies of the old characters. For example, there is an alien who looks and acts like Carl Wheezer and even has the same voice as him. Why couldn't Carl go to Zeenu with Sheen? Sheen dates Libby in JN, but he dates a blue alien here for some reason. What happened to Libby? Did Sheen break up with her? Throughout the series, Sheen tries to get back to Earth, but I heard he decides to stay on Zeenu in the series finale, which makes the whole storyline pointless, though I've never seen the series finale.
Don't watch Planet Sheen. Watch Jimmy Neutron instead.
