with me it depends upon the person I am speaking to, as sometimes it makes people listen to what I am saying to them. I work in education, but am not a teacher (of children anyway)where letters and titles count for a lot. It sometimes makes me smile when I pass the head's office with its plaque detailing the qualifications they have as if I had one on my door they would need to make it bigger to get them all on . People in education seem to like this kind of thing, so when I have to deal with them I just casually drop the fact that I am studying for a PhD into the conversation, and like magic they stop treating me as though I haven't a brain cell in my head, which sadly happens all the time if you are not a teacher.. The rest of the time I only mention it when I have to have an hour or so off to see my supervisor!