I've been using blog search for a while but in a way thats different from normal searches. Blog search engines are good for an ongoing search. Take a topic you're interested in. I like reading about system administration or shell scripting for instance. Search on this topic on technorati.com, or google blog search. The results are available to you in RSS format. Subscribe to this feed. Now you have a way of going through the blogosphere and reading about topics that interest you.
Blogs generally don't stay on topic. One post might be interesting so you subscribe to the blog, but it turns out that the post was an anomaly. A topic not to be discussed again. In the mean time your RSS reader gets full of blogs whose content you don't really care for anymore. Through blog search you have content that is on topic.