I love instant messaging as a mode of communication. Seriously, I think it is my preferred mode--above email, phone calls, maybe even in-person conversations! I like the brevity, the almost-but-not-quite instantaneous response (the pauses giving you just enough time to think of a better or funnier way of saying things while still maintaining an actual conversation), the ability to have multiple conversations going at the same time, the ease of adding pictures, links, and quotes to the conversation...and of course in an argument, you can always scroll back and prove that the other person actually did say what you are accusing them of saying!!! It's a beautiful thing, really!
Of course, with any new communication technology comes a period of adjustment while norms and etiquette are established. Recently, Tina posted on scatterplot about one such problem--people dropping by your office and interrupting your IM conversation as if it isn't a "real" interaction and expecting you to immediately drop your cyber-chat for the face-to-face variety.
A related problems I've often experienced is that the person with whom I am IMing sometimes seems to forget that I exist in a whole other (not nother, other!) world beyond the IM conversation--which can, in fact, interrupt the conversation and slow my response time. If I'm IMing at work, for example, it is indeed possible, and in fact extremely likely, that someone will pop into my office, unannounced, and ask me a question, or two, or three. Or, that old archaic phone might just ring. Or someone else might pop in with an IM question (we use it a lot in our inter-office communication). At home, I have kids. They demand attention like no other beings on the planet--and generally speaking, whether it is a scream of pain from the kitchen, an IM from the living room, or a cell phone call from the basement(!), I respond.
I've noticed negative reactions to these pauses, of course, more from those who are newer to the technology. Just like people who haven't mastered the cell phone side conversation process or can't accept that cell signals drop during even the most stationary conversation, people who are still trying to map a face-to-face conversation onto IM are more likely to be offended. Those who use IM a lot have become accustomed to its advantages and disadvantages and seem completely unflustered by inexplicable (and inexplicably long) pauses and resumptions of conversation, with nary an explanation needed. Now, if I can just remember to turn that darn thing off when I'm leaving work so people aren't talking to an actually empty room....