I have been using Twitter for a few months now...and am still learning. My first exploration of Twitter began after a workshop (on open source management systems) where a number of participants were tweeting and using the hash tag suggested. Although I refrained on the day - looking at the entries on that hash tag later encouraged me to dip my toe in the water.
To start with I followed some of the people who had been at the workshop - and looked at who they were following - and so on. A hint I was given - which has made all the difference to how much I use Twitter, is to be ruthless with who you follow. A couple of the people I followed early on, tweeted many times a day about personal trivia that one had to wade through to get to the gems. Now I no longer scroll through pages to find what I want.
I confess it is only in the last few weeks that I have discovered that clicking on the @libchris on the sidepanel brings up all those tweets where people have mentioned me in their tweets! Wanting to explore Twitter further for this 'thing', I have now personalised my page as well :) (and of course chatted to other Cam23 participants)
So what do I use Twitter for?
Largely for information gathering - If you choose who you follow with libraries and information in mind, you can find a wealth of material.
From the other side - it can be useful for disseminating information - one can reach a wider audience because of the links of someone who follows someone, and the facility to retweet.
In a work setting - I can see that it very useful for information gathering - for keeping oneself abreast of developments. Also for general 'advertising' it has a place. However it's very fluid ever-changing nature means one could not rely on it on its own to get information across to one's followers, as I suspect few would scroll down all the posts since they last logged in. (unless of course they constantly have a Twitter window open). This is also assuming that the library's users choose to follow it in the first place.
So - as a personal tool for CPD and keeping in touch - I think it's great. As a means of keeping users abreast of developments in their library - it has possibilities, but only in conjunction with a website, e-mails etc.