Before I began to use Twitter I imagined it would be a total waste of time. Indeed, if you read comments that truly answer the question 'What is happening?' as it suggests, it will be. Who wants to know if career-Mum is giving junior his toast? But many people just use it as a way of expressing wisdom, informing people about useful and interesting things, and so on. It is also a userful marketing platform. Personally I find its 140 character limit means you can make a worthwhile post with minimal interruption to your normal work. This constraint should work well for authors and it might even be a useful tool to help break writer's block: for which, just writing, is the best cure. Indeed, who is better suited than writers to be able to construct something interesting given these stringent constraints? It helps you to become concise.
When writing, I can get to a scene ending and then make a post. It is light relief. Why bother? Because it potentially brings you loads of followers and, as a writer, what more could you ask? In just under 2 months I have over 1,900 followers. I put in around 90 minutes a day. Okay, they don't all translate into sales but, given time, some may buy. Even if they don't, it is a good way for a writer to raise his/her profile. It is much more time consuming finding something worthwhile to post in a blog, which is why I now Twitter much more than I blog. Please check me out on Twitter at http://twitter.com/#!/authorkingsley
Don't just use Twitter as a sales tool, though. Okay, you can mention your book, or whatever you have to sell, but you need to regularly post content that is interesting in its own right as well or you will soon turn off potential followers. My best advice? Target/follow users in your niche. If you don't follow others they won't follow you. Good luck!