It was... surprisingly good. I was sucked in by the casual nature of the show. Short episodes that you didn’t really need to pay attention to with some super dry, dark undercurrents. As it progressed, however, they started to weave a complex cannon and mythos that was actually super interesting.
The darkness of the Great Mushroom War (nuclear apocalypse) mixed with the apparent cheerfulness of the series... good stuff. The whole Ice King/ Simon plot was... really sad and complicated and deep. The whole Lemon Hope plot also really got to me.
The latter seasons went for big story arcs and really made an strange episodic cartoon into something... more.
The final episode was extremely well done. They pretty neatly wrapped up everything that was going on at the time, plus gave some conclusion to almost every significant one-off character. Also yeah... the same-sex kiss was extremely well handled. I also didn’t see it coming but apparently I’m like... the only one.
Overall, even as an adult it was a really good series and I’m glad I watched it.