I see this being especially useful for the site helpers such as Diablo II helpers, SA helpers, etc; and also the staff team. As for the normal user, I guess I can see it being useful. Like for trades - FT replies and ISO replies. There should be a limit though, on how many categories one can have. Three seems reasonable. And I agree with you, there should be customization, because a certain user might get a surplus of one topic more than other topics - customization would become useful here. But yeah, it'd be nice to have some organization in PMs. Normally I just tend to my PMs as they come, because I'm on so much, but it'd be useful for prioritizing which to reply to first.

Voted yes.