One of the questions that I get asked most often is if authors should have an author blog--a professional blog for them to talk about their books and writing. Many, if not most, of the authors who I work with do, and for the most part, I think that this is a great thing. Blogging seems to be the queen of new media--authors promote themselves on their blogs, take blog tours, and the myriad review blogs opine daily on the latest releases. Blogging gives authors a platform that they have no other way to get, and it allows them to cultivate a readership who cares not just about the author's next release, but about the author themselves, and I think building that sort of readership is a great way to start a career.
A hundred readers who are passionate about your work are, in my opinion, more valuable than a thousand readers who buy your book on a whim and don't feel strongly about you either way. The hundred readers are, obviously, fewer immediate sales, but they're the people who will say to their best friend "Oh, you just have to read this book." They're the people who are going to ask if you can come speak at their writer's group, the people who are going to buy every book you put out. This sort of fan base is invaluable to an author, and having a blog is a wonderful way to build it.
There are, of course, pitfalls. One of the things to keep in mind is that everything you're posting is in a public forum, and it can--and, let's face it, probably will--be found by other authors, potential fans, and editors who are Googling you. When those people, all people who have the ability to affect your career, search for you, you don't want the first thing they find to be a flame war between you and another blogger. You probably also don't want your furry fanfic showing up on the first page of results, or an angry rant about a bad review. The last thing you want to do is put people off before they've even opened (or bought) your book, and part of that means that you need to present a professional front when you're in a public space like the internet.
If you choose to blog, even if you don't intend to use it as a professional tool, you should be aware that the people who find it are often in your profession and that their opinion of you has the ability to impact your career, for better or for worse. When editors get a new submission, we'll run a search for your name. While I don't think that anyone's going to reject you for your blog, it makes us wary if you're complaining about the utter stupidity of one of your editors, raging about low sales, or otherwise betraying a naive view of publishing and an unprofessional demeanor.
Sometimes it does come down to what everyone's mother told them on the playground: If you can't say anything nice, it might be better to say nothing at all.