OK - for those of you who know when I'm at work, yes, 99% of the time I am on. If I'm running invisible, it means that I don't even have the 10 seconds every 5 minutes or so to close the window that pops up when someone messages me. Either that, or one of my bosses (re three quarters of the whole company) is around in the room &/or I'm not in the room &/or we have (scowls) visiting guests who complain about anything not work related. Email will work almost as well if you want me to get a message, and won't pop up and get me in trouble.
running around at work; will reply soon means I can get messages and normally have a little time to reply every 10 minutes or so
doing mail means at desk, but rushing to get someting done. Messages will get read but not replyed to until the project is done.
everything else without a busy icon means I'm ok to talk; I just hate the whole I'm Available bit :P
As far as at home, I try not to run invis if I can talk. It's really that simple.