I think there was a Colbert Report episode where Stephen said we could save the African Elephant if we all just went on Wikipedia and edited it to SAY that they were no longer endangered. And then, if you blogged about it and he blogged about it, any search that someone did would give three sites saying that the African Elephant was no longer endangered.
That's how I felt when using these blog search tools. I'd type in subjects that popped into my mind and got several blogs with similar information. I wasn't sure how much of it was true, but it was clear that all the information had come from one site and these bloggers had wanted to share the info/links with their readers.
I find the trick to navigating the Blogverse is pinpointing the blog that gets the news out there first. Because there are billions of Harry Potter fansites and Star Wars related blogs, subscribing to more than 2 of them would be silly because, 9 times out of 10, you're probably just going to see the same information twice. So, you should look at other people's blogs to see who is getting the information you care about out first. (This is why subscribing to site feeds can get messy - sometimes they update too much with very random news items. If you can find a blogger out there who is already sorting through that stuff and then re-posting it in their own blog, that might be the best way to go to keep from getting information overload).
And this isn't just specific to pop-culture blogs. I had a couple library related blogs that I subscribed to, and more often then not duplicate information would pop up. It got to the point where I decided to just take it down to one feed because I was tired of scrolling through the same news twice.