The show started out as an okay-ish parody to Johnny Quest but by the later half of season 2 evolves into it's own beast and season 3 really kicking things into high gear with great over arching plots, subtle character connections and back story drops (some of which are VERY subtle and don't pay off for a season or 2) and some really good comedy moments. The way characters grow and change is also pretty fantastic, by this last season pretty much every character in the show has gone through some major growth and are very different from where they started out (except maybe how pathetic they are sometimes) and that goes for how they interact with other characters too.
If you want to get started I fully recommend watching it all, but there are also catch up videos up to S5
Spoilers Obviously: