I don't really mean that this has to be an either or situation but that seems to be the place many librarians find themselves at this point in time. We've invested in products like Question Point so that it may seem like a step back to consider also offering reference assistance via instant messaging.
The advantage to communicating with patrons via IM tools like Google Talk or AIM or Yahoo Messenger is that some of your library's patrons (or potential patrons) are already familiar with and using them on a regular basis. For these individuals the chances that they'll be willing to IM you a request for information is probably greater than the chance they'll go into Ask A Librarian. That might appear daunting, particularly if the question is a simple one like "Are you open on Saturday?" Or "Do you have any graphic novels?"
As Bruce said yesterday, we need to get into the habit of saying yes to more things. So let's say yes to IM. And for those of you who've veered away from it's use in libraries because you didn't want all those obtrusive programs on your public access computers, there's an easy web-based solution meebo.com. You or your patrons can use meebo to check multiple accounts. It's easy and certainly more secure than having individual contacts and buddies and messages stored on library computers.
All of these IM accounts are free. Most come with advertising but that's the way of the world. Here's where to go to set up accounts on the big three: