When I have to phone a colleague at work, I first look in Windows Messenger, whether he is at his place, i.e. online. I expect that the Messenger changes his state to offline when the user's not active for a specific amount of time. Well, some users either don't know about this feature (what's half excusable), or they don't like it (why?), or it doesn't work for them (@MS: why?). Altogether, this feature is useless.
PS: Yes, I'm using Messenger Plus!