On Jesus Creed, Scot McKnight has a great post on Blogs: A Word for Authors, with a strong prohibition: "If you have written a book, don’t create a blog to market your book". I have been asked the question by authors too, "I'm thinking of starting up a blog to get some feedback on my book", etc., and invariably the person asking the question is not a consumer of blogs, something that is unlikely to lead to successful blogging. If you solo-blog, i.e. don't interact with any other blogs, you simply won't get much attention anyway, unless, I suppose, you are a huge name. An N.T. Wright blog or a Bart Ehrman blog would, I'm sure, get some attention.
One qualifier, though, to Scot's remarks. It is quite possible for someone to begin a blog in connection with a publication, and then to move on from there. And that is one of the keys of successful blogging, to let your blog evolve. I may be wrong, but I think that James Tabor's Jesus Dynasty Blog began as a venue for information and discussion on Tabor's book The Jesus Dynasty, but it has evolved into a blog that discusses a variety of other issues related to Tabor's research, and it's one I always enjoy reading.