My username on all clients is "chronarchy". This is not a coincidence. My ICQ number is 9951447. You can send me an IM or leve me a message any time you wish.
I may or may not respond, though.
You see, the only time I use IM is when I'm at work. I have it on, actually, for work purposes. The guys downstairs at the help desk IM me. The students at the front desk IM me. My co-workers IM me. Thus, you can get ahold of me pretty much any time during the day (8 AM to 5 PM Eastern) with no problem.
If I suddenly stop answering you in the middle of a conversation, or if I do not respond to you, that does not mean that I am ignoring you. It means I'm with a customer, or else engrossed in actual, real work. Quite often I can work and IM at the same time: I'm good like that. But occasionally, my customers require my full, undivided attention, as does my work. At those times, I ignore windows that pop up, or I leave a conversation completely.
Will you notice me leaving? Only if you're very lucky. If it's something planned, like a meeting, I will probably mention that I'm leaving.
If a customer walks in, or my boss pokes her head in, or if I take a call, I will just stop responding. I don't have time to type "brb". Quite often, I'm halfway through a response when that happens, and most times I'll be back within 15 minutes and will finish a sentence as if I never left. Sometimes, it can take hours, if I'm pulled away from my computer and then into a meeting.
Early mornings and late evenings are the best time to get my attention. Middle of the day is often bad. At about 1 PM, I become rather busy, and this goes on until about 4 PM. I'm also pretty busy between 10 AM and noon. 12:00 to 1:00 is my lunch hour.
In short, if I don't respond, I'm not ignoring you, and I'll get to you eventually. I treat everyone equally when it comes to IM. Either I'm answering everyone, or I'm not answering anyone.
Now, on to who I choose to IM. The answer is simple: nobody. I almost never initiate an IM conversation unless I need to get ahold of someone for something. In fact, I don't even look at my buddy list to see who is online: I assume that if someone wishes to talk, they'll IM me. This works out best for me because I dislike initiating an IM conversation and then dropping it. Again, I treat everyone equally.
BTW, if my AIM ever goes to "available", you can certainly feel free to IM me. I have it set to "away" all day because I am, technically, not really available. But you can feel free to chance it any time you wish.