Peter Davidson has a post worth reading titled Thinking About Product Blogs.
‘Many product blogs(PB) and small biz blogs(SBB) fail to define narrowly enough their audience and what they want to accomplish with their blog. Are you writing for existing customers or prospective customers? Both is the common answer. I would say this is a problem since the interests and information needs of these two groups are very different. For example: many SBBs and PBs post every media or prominent blogger mention they get. While this may be useful to people seeking additional information about your product it is not very useful to existing product owners. Do I really want to read all the press clippings about a product I already own. Press mentions should be compiled in a sidebar or miniblog.’
Read more of Thinking About Product Blogs.