A subtle touch, I think. A few words here and there. You can go overboard with it (coughcoughLovecraftcough), but even that can work if done well (c.f. Mark Twain). Some people say you shouldn't ever use phonetic dialects, but I think they're fine in certain circumstances.
I find them most useful when distinguishing between various types of character, though. That is, if everyone in the story is from the same odd location, then giving them all intense phonetic accents is a bit weird.