I like the idea that we can PM each other, though to me it can be as intimidating as approaching someone I don't know and attempting to strike up conversation in real life, which I'm too shy to do.
When I was new to the boards I remember sending a pm to a member who had given me a reputation point, thanking them for it and complimenting them in turn. Though it was well-received I remember that curiously this member (of the opposite sex) casually mentioned that she was already satisfied in a committed relationship. This surprised me as it seemed to be mentioned apropos of nothing and so I got it in my head that pm's in general might not be quite so casual as they initially appeared. Though I'm now convinced that this is not necessarily the case throughout the boards, it did increase my skittishness about sending pm's to people, and consequently I rarely receive them myself.
That aside, I do like the fact that we can send private messages. As stimulating as the discussion forums can be, I find that in spite of the abundance of shared opinions and confidences, there's something that is still quite impersonal about them as though we're all pouring our thoughts into a void. Although there's plenty of interaction and banter, imho there still is a tangible lack of real connection that is not satisfied in all the agreements, disagreements, and general contributions of each respective member's 2 cents.
As I've come gradually to this current outlook, and as life has gotten busier, my participation has dropped off. At the same time, I really don't know how I would initiate a conversation via pm anyway, or what the hell I would want to know about my fellow members in the first place. Either way, I think it's good that the option is there.