but many other people have already written about the differences between them:
The emphatic quotation mark in Japan
(I think the Scripting Japan blog or the blog's author on Twitter mentioned how multiple kagikakko can be used to indicate multiple people saying the same phrase in unison (an extra pair for each extra person. not in formal use, but I see it now and again in popular culture/media), but I can't find the URL. This point is also mentioned in the second link given above.)
i only wish the message would get through to some of the clients I have to deal with...
i might just bang out the post anyway cause i'm having a fun time writing it