According to the
RAE, "leísmo" is accepted when the direct object is a man. ("Le quiero".)
When it's a plural --"les quiero"-- its use is advised against, and it's considered plain incorrect when it's an inanimate object.
When it's a woman, "le" is accepted in impersonal sentences --"a mi abuela se le quería mucho en el pueblo".
There is also a "leísmo de cortesía", which is used for people to whom you call "usted":
"Disculpe, no le había visto; ¿le puedo atender?"