Just like PPG, it went downhill.
Plot: Billy and Mandy, two kids who are best friends. Billy is a kid with mental issues, while Mandy is an diabolical, sinister, dark girl. They live in the city called "Endsville" a city full of super naturals, and celestial beings. One night, was supposed to be a simple night for them, until Grim Reaper, came into Billy's (along with Mandy) house for a stop. Now, Grim, thinks that those kids can be scared of him, but he's wrong. He can't take those kids around, because of Billy's stupidity that it can foil Grim's schemes, and Mandy, is too smart for Grim. Now, they took him as their best friend and sometimes, a butler.
Characters: They are nice, quirky, and crazy in a good way. They really move in places that you don't know, they are unpredictable. Until the last season of course, they went too simple. And simplified their agendas. Voice acting is nice, and the dialogues are also nice.
Humor: Dark, creepy humor is always used in older seasons, and they can use it well and gives out good laughs! In the last season, overused gross jokes, they went with fart jokes, and yo-momma jokes. And some jokes, plots are reused from the old episodes.
Art: In the 1st season, the quality of art and animation is good, they made it better afterwards. Their improvement really gone well, it hit the nail on the head. No mistakes in the last season was made in art.
Overall: 8.5. This show would get a 10 or 9.5 if it wasn't ruined and sunk in the last season.