I want to give this a 4.5 because of course I'm very biased, but I also hope for more AT universe to be created.
This spin-off does stands by itself and new viewers can enjoy it, maybe not with the same amout that (us) fans will, but it's really nice that the writers put the care to make it newbie friendly.
The story was good, seeing Finn and Jake was awesome, and following Simone in a redemption arc was such a treat (as also seeing a bunch of other iconic and not so iconic but great characters in cool situations or even die, well, that was really unexpected and appreciated).
At some points some characters were a bit cheesy, like Cake's dialoges at some points or the main villain at times feeling more goofy than evil, and there where some deus ex machina at the end to reach a happy ending that I'm not so sure actually make sense, but, hey: AT never made sense at the beginning at it was still awesome. I just hope that if more spin-offs come they try not to take the easy route (maybe this was impossible to do this time as it was the only chance they had to give Simone a happy ending, but the original ending of AT was nontheless bittersweet and great, and the other one in Distant Lands was just... uff!)
A must watch, even more if you are a fan.