Opinmind is not another blog search engine since it helps you "discover and connect with people with similar hobbies and interests". (although it walks and quacks like a blog search engine ;-)
One interesting feature is how it categorises results between positive and negative opinions with a "sentimeter". I typed "bird flu" and the positive results came up with "love", "good" or "awesome" in the sentence while the negative have "hate" or "sucks" or "stupid". It looks like the sentimeter matches keywords according to their proximity in a sentence. Probably pulling from a dictionary of negative and positive qualifiers. This works well for opinions which are expressed “directly” but could miss more subtle comments. There is also a risk that one whole post will be judged based on one sentence. i.e. the whole post is positive but there is one sentence containing "hate" and the term you search for too close together).
Nonetheless, I think that it is great to have a tool like Opinmind freely available. I am sure it will grow and improve with time.
You can read the Opinmind blog here.