I've tried out a number of the web communication tools in Thing 7, and look forward to trying out a bunch more (especially as our two weeks of spring break are fast approaching, giving me a little more time) but wanted to get this up in the meantime.
We use FirstClass for the school email system: it not only provides email but a number of other capabilities (forms for building needs or computer support requests, a calendar, etc.) It's not the most robust system in the world, and it has some quirks, but it works well for internal communication.
There are a couple of frustrations with it, including the fact that it's hard to filter emails into folders (it's turned off on our accounts, anyway), which makes it hard to keep up with professional email lists, and to sort out messages to me from messages to the list and to read one or the other quickly (these days, I run most of them through Gmail, which makes everything easier). It's also got some weird quicks - you can't use a keystroke command to bold or italicise items, which makes writing some kinds of emails more tedious than they might be. (For example, anything involving a title)
How do we use it? Our media staff do a lot of face to face communication - but often someone will be involved with a class, or not available for some reason, so email is a great way to drop a quick note. Teachers use it for questions about computers, and I send out an email reminder every night to teachers with what labs/laptops they have reserved for the next day. (This is a bit of extra effort, but we're getting far fewer people forgetting or getting confused, so it's worth it.)
We don't currently use online reference at my current job at Blake - there isn't much need for it, and students with computers are likely to be within easy walking distance of us. I did get a chance to use it while working at St. Kate's in January, though.
I really like the ability to respond to someone online, and for them to ask questions in a way that's minimally disruptive to what they're doing. (For example, they don't need to disturb others in the stacks with a phone call, or to wait on the phone - they can go back to what they were doing, and when I have the answer, they can come back.
I did a bit of reading in the professional literature about it at the time, and do agree with concerns about how you approach it (for example, introducing yourself, especially if there's a generic account), and in terms of letting the person with a question know what you're doing while you're not responding (i.e. "Ok, let me go find that for you. It may be a minute" rather than just being silent.)
I've been active online, including in real-time text conversations, for over 10 years now: there's a definite art to communicating well. Some of it comes from basics like typing speed (someone who types 80 or 90 words a minute is going to have fewer gaps in conversation). Some of it is being able to adapt to the individual presentation.
For example: can you read 'netspeak? I can, but I have to work hard at it, because it's not used in the places I spend time online much. What are the polite ways to ask someone - who may be nervous about librarians anyway - to explain what they mean better? Obviously, many of these issues come up in face to face reference settings as well, but you don't have things like body language to help make it clear you're just trying to help.
What I use:
I have a hard time with IM and text messaging when I'm focusing on getting work done (whether that's at home or at work). I also have a hard time being concise, which means I work hard when I'm talking to people who don't know me, to keep things short and simple.
However, I do enjoy using it for quick messages, or to touch base with friends, and if I'm home and not doing much besides web browsing in the evening, I'll often have it on.
It's also been a lifesaver to me when I have a big project that involves a lot of computer grunt work. For example, I help put together the collection of student speeches each year, and it almost always involves a weekend day at work doing nothing but the formatting and copying and pasting into the final format. Having IM on helps a lot with the boredom: I can chat with friends while I'm doing it, and sometimes get an idea for problems that come up.
I've also seen it used for quick questions - the kind that need a response sometime sooner than later, but that aren't urgent, or that are more easily done in an online reference. The "Is this the wording you wanted for this thing?" or "Hey, I see a difference between this page and that page: which did you want?"