Boxfish/cowfish are cute and funny when young, but then they get big and ugly. They can also excrete a toxic substance if they get scared enough. This can kill things including themselves. Also they can be a challenge to feed.
I bought one on impulse after having a margarita at my local mexican restaurant. Then I learned about all this stuff and ended up trading him for a sailfin blenny and some snails.
If you can get through all of the challenges they are pretty funny to watch. They like to go up and down like a small blimp. Other than that I do not think I would ever get another one. 8)