Twitter is undoubtedly my favourite thing so far. We have been using it for a couple of months to promote the Polar Museum @polarmuseum and are rapidly heading for 200 followers. I've been amazed at how rapidly a following develops, but having the University of Cambridge retweeting us regularly has helped. You can follow us at http://twitter.com/#!/polarmuseum.
I've been using Hootsuite to schedule our Daily Polar Photos (great for doing several weeks worth at once) and also love its ShrinkLinks feature, having used both bit.ly and ow.ly in the past. It's also interesting to see how much faster it is to post stuff using an iPhone rather than my desktop (maybe time for a new PC?) None of this would have been half as easy without our very own Twitter Tsar (or Chief Twit, as she claims to be known). Having an under 25 on the team means instant access to someone who knows their way around social media.
Twitter is easy enough to be addictive and is one of the tools that has had an impact professionally. It's important to keep an eye on the feed, however. We get occasional questions or comments from followers which need a reply, so if you do start using it to promote a service, I'd suggest ensuring that you have a team prepared to provide cover.