I loved it. I liked how they constantly referred back to episodes of the show without it feeling inaccessible (Mickey's backstory, Felix's homicidal girlfriend (and his treehouse), The One-Eyed Snakes, etc).
I did think the identity of the murderer was fairly obvious from the start, considering the screentime given to such a minor character as Grover. I didn't even realize he was a pre-existing character until looking it up right now - he was in Roamin' Bob-iday in Season 9, and Copa-Bob-bana in Season 11, which would have been probably produced after the movie was written, as it was originally scheduled for 2020. I'd have liked for them to have added him in to a few more recent episodes, just so that he didn't stand out so much (and I like David Wain).