Owning a blog requires you to answer some fundamental questions even before you begin. For example, questions like will you allow comments on you blog? If you do allow comments on your blog, then what should the comment policy be? Clearly, it is easy to that there are no easy answers to these kinds of questions.
There are arguments for blocking comments entirely. It saves lots of space and time. Managing and responding to comments can get you a lot of valuable feedback from customers, but it can also eat up valuable time which you might not have. If all you're worried about is getting a post up there for SEO, then comments may not be necessary. Also, if there is a fear of negative comments, it may be worthwhile to start the blog without comments, since it's easier to add them than it is to shut them down.
If you have the time available to manage comments, this is usually the better option even though it can be hard to keep up with comments. I haven't responded to the comments on my past blog entries yet, though they've been great to read and I intend to get to them, however, much of the reason to blog in the first place is to connect with consumers in a new way, which can't be done if they don't have a voice on your blog. Blogs are all about community and the exchange of ideas, not one voice talking to fill an empty webpage.
Positive comments are great to send around the office to show customer appreciation as they reveal when you are doing things right and are really helping people. If you run across a particularly satisfied customer, you can ask them to share their experience on the blog and this unbiased feedback can give rise to new customers. There are many advantages to having these kinds of responses.
As far as negative comments go, I think it boils down to remembering that these opinions exist somewhere on the Internet anyway. They will find a way to make themselves heard whether it's on your blog or elsewhere, however, if it's on your blog, you can respond directly and engage in the conversation. You can explain what you agree with and what you don't. Most importantly you can let them know what you are doing to fix the problem, whatever it may be. Negative comments, as long as they are constructive, can actually be an asset and can go a long way towards improving your product and increasing customer satisfaction.
This brings me to the comments that simply are not constructive and, hence have to be dealt with. Before you begin blogging, consider creating a comment policy. You can include it in the comment form page or on the sidebar. This way, everyone knows exactly what the blog is about and there is less chance of a backlash from people whose comments are excluded. Usually, this involves deleting any spam comments, foul language, and personal attacks. This can be done by moderating comments and having to approve them before they appear on the blog or letting all comments go up but removing those that are not appropriate.
In the end, each company has to make up its mind in regard to the comments question. As long as you're clear and consistent about your decision, this is a question that doesn't really have a wrong answer.