The Fundamento defines ci and cia as follows:
ci tu, toi, | thou | du | ты | ty.
ci'a ton, ta | thy, thine | dein | твой | twój.
Unfortunately, I've never actually seen ci used anywhere. I've seen it described as impolite, but most of the explanations as to why it's impolite tend to hinge on English "thy" and Russian "ты" being considered sarcastic or overly personal in most contexts.
However, I think these explanations might be oversimplifying the issue; this is Esperanto, and the connotations in other languages wouldn't necessarily carry over. Exercise 16 in the Fundamento contains the following line, suggesting not to use ci, but without any explanation as to why:
Ci skribas (anstataŭ „ci” oni uzas ordinare „vi”).
This does however suggest that ci didn't fall out of favor, and was never a common usage. Is there ever a good reason to use ci? If so, in what context, and if not, why?