I do get the theory of Twitter; I have a username, I've "tweeted" (hate the cute terms) and so on. But there's no real point as no one else I know uses it. Those who have usernames do the same as me, and use it at conferences and so on.
When I say I get the theory, I mean that I like making my current status available, and seeing others. How? Simple, IM already has a status line:
I update this all the time, and people actually read it. I get comments directly back on the status several times a week. If I forget, and it just says "available" I'll more often get "so, how's it going?" IMs, just to keep contact.
IM, and even better the IM client that connects to several systems, means that I can do this with almost everyone I know, without them having to subscribe to some new system or process.
While awful in every other way, the corporate IM platform we used at Sprint (MS made it) automatically changed the status based on what your calendar said. That was super-convenient, as it would have both the standard auto-status (e.g. offline) and with additional data (in a meeting). I'd love to have an IM plugin that detected location, time, etc. and said what I was likely to be doing as well as what I type in.
Next I should set up the IM client on my S60 so it works well and actually do all this even when I don't have a computer in my hands.