Mystic Onyx
Personally, I didn't care for it. Should've been named "Watch Finn Fail."
No point to him being in the episode, really. Outshined by CB the entire time.
Also, CB without poo brain is...eh...I don't like'm.
Plus, we already know Finn and the princess(es) are over, so reinforcing that seems unnecessary unless it was done for future plot material.
while we're on the topic of future plot material, there previously was an episode scheduled titled "Frost and Ice", which most likely would be a continuation of the whole finn-FP debacle. this was removed from the list of scheduled episodes, however, but it wouldn't surprise me if it resurfaces sometime in the future.