Blogs are a chance for ordinary people to put their voice out into the world. It's a chance to say, 'this is what's going on with me, this is what I think and I feel.' Blogs are meant for personal expression, not expert opinion, so when experts and journalists enter the fray, the results can be a little confused.
An example of a good blog is Washington Post columnist Dan Froomkin's blog about the White House. Because Froomkin has an interesting life and interesting connections, reading his blog is informative and reveals interesting tidbits about White House correspondence that don't have any acceptable place in a newspaper. Kevin Maney's tech site is another example of a blog that just serves as a repository for things that don't really belong in a newspaper. This, I think is the proper role for professional journalists' blogs.
Unless, that is, you are someone really interesting like Kevin Sites, the photographer who captured the Falluja mosque shooting. He has a fascinating blog that serves as a boon to his employer NBC, because it creates an attachment to Sites as a person and a sounding point for him to rebut his critics. His blog, though not perfect, is interesting just because of the type of person he is and the situations he gets himself into.
But most bloggers (e.g. Leslie Kelly and Janell Hazelwood) are, as the French would say, looking at the world through their own bellybutton. That is, they only talk about themselves and don't have much more of value to add beyond their articles.
Part of this is certainly due to the fact that these journalist-bloggers have demands on their time from those who pay them, and blogging "don't pay rent." Wired magazine's "Heartaches of Journalist Bloggers" laments this fact, but that's just how it works in a capitalist society: intelligent, thoughtful analysis by informed individuals is something you pay for. Man-on-the-street interviews, even in their present incarnation on the Internet, are useful in gauging public opinion, but shouldn’t be relied on for reasoned and informed analysis.
Which is not to say that personal blogs are useless. Global Voices has a list of blogs from around the world that are useful for discovering what people in places of interest around the world are thinking about. In this regard, these blogs are much more useful than professional journalists who may have just “parachuted” in to cover a story and who have no idea what is actually going on in the streets.
But overall, I think it will take time for this new medium to decide its true usefulness to humanity. Blogs can be used for many things, but the popular ones will be those that offer uncommon information, a unique viewpoint or an interesting narrative. Not everyone (including we journalists!) can offer that.