I've only used this once and it was when there was a problem member online on a forum I help on. I was trying to see if he would cause trouble when no one was online too.
I've used it quite a bit, but not too much anymore. It comes in handy when you need to catch up on PM's or just want to lay low for a while without having anyone bug you. 😛
I never use it on forums I am a member of as I find no use for it, but I have used it on my forum before. We had this long bout of problematic members/trolls and when it looked like I was not online some of their antics stopped and they wouldn't PM/email/etc me. I see that occasionally some members of my forum use that feature too.