I think Finn and Jake were good until the end. When the show wasn't casting them aside and still giving them episodes where they were the focus they were often still good. I liked First Investigation a lot, and .... fuck me is that really the last episode in the final season focused on them? There was stuff like Gumbaldia and shit but I didn't find they were the focus there.
Anyway, when they were still given time to shine I still liked them. However the show was often utilizing them in as minor ways as possible and not giving them enough good screentime or focus- the finale is the worst example of this where I didn't find most of what they did do to be compelling and they were fucking separated for most of it.
Imagine being told the finale for Adventure Time barely had Finn and Jake together throughout, what the fuck.
Finn and Jake were good enough to the point where I still considered them good characters but the show they were part of wasn't as often interested in showing that. My poor boys.