I agree with all above, dear is often used but my dear less so. My dear is very old fashioned, to support this, I think I really only hear my dear used by pensioners often when they speak to younger people, going through a supermarket till for example or maybe when talking to grandchildren.

Another point of view. I generally see it at the start of written letters and I seldom heard it spoken. "Dear" is used in speach and to anyone close but more likely to a woman (I'm male) but "My Dear" sounds like the start to a sarcastic remark. It might be regional? Certainly greetings like "me Ducks", "me Love", "Darling" and "Dear" are used in shops to customers and different ones are used in different regions. I was recently addressed as Darling in a shop in Yorkshire (where I live) by a woman who clearly came from London and I had to stop and think "why am I so annoyed?" I guess it felt inappropriate and I calmed down and explained myself to the staff member.

While I mostly agree with David's summary above, I neither use it, nor hear other people using it, except possibly in a camp, jokey kind of way. Dropping "my" and just calling someone "dear" is a little more common, but "my dear" is a bit old-fashioned and not generally used much in my experience.

It depends. Here's an overview of how it is used: Firstly, it isn't strictly between gender and the opposite gender. -naturally, a boyfriend/husband can say it to his girlfriend/wife. -One can also say it to a friend. They don't have to be very close friends. It does imply a level of closeness, though. I would say it is perfectly OK to say it casually to someone you see fairly regularly after a few months. -it is also used as part of a slightly camp vernacular to almost ANYONE. (Excluding formal interactions) As a way of example: I use this with: almost all female friends unless we have only just met. I use it with female relatives. I use it with CLOSE male friends., though almost exclusively like this: 'how are you my dear?' And if I had children they would be addressed this way too.