While I acknowledge that the theme might not be appealing to everyone, and it is a "silly" game, I still believe that CFA is the best designed sandbox RPG available at the moment.
Why? An awful lot of sandbox RPGs present sandbox settings but with a very limited number of eventual goals which limit engagement. CFA, and The Big Pie Caper in particular, take the opposite approach: the players are flooded with different objectives and neat things to do, all of which can shape how things happen in the future, and all of which contribute to the "mischief mote" system that drives play.
On top of that, to prevent stagnation via PCs having a limited number of "best" plays, the costume system acts very similar to the Cyphers in Numenera and those system - it gives the players constantly shifting roles and abilities, but with a greater degree of control over them. These two together to work brilliantly within Chen's Flow theory to allow players to influence the game into whatever's most enjoyable for them without needing to break OOC in order to do so.
No-one I've known who has played this game has disliked it. Highly recommended.