Alex compares these by feature and by functionality–that is what particular circumstances you’d probably want to use which search engine for. I would agree that this would be the correct way to compare–so to speak–the various blog search engines, since this is mostly like comparing apples to oranges. They each have their strengths (and limitations), and they each have their own way of organizing the vast amounts of blog content out there.
Technorati does great at pointing out the popular topics and tags of the moment. Google Blog Search does just that: search. Ask Blogs & Feeds is strong in organizing using various criteria. Sphere helps you find related content. Icerocket is sort of a combination of Google, Technorati and Ask. Findory, meanwhile, recommends content for you based on your clicking habits.
So if you want to have some idea about a feed and its popularity, try Technorati. If you want good results and a minimalistic interface, go to Google Blog Search. To preview search results, use Ask, and to find related posts, Sphere could help you. Icerocket tries to be a bit of everything, but it only partially succeeds. Findory shows mostly popular blogs, but it’s a good filter if you don’t want to read too many posts.
Of course, this is just the tip of the iceberg. There are other blog search engines out there, and you can even include the internal search functionalities of social networks that provide blogging services, like Multiply, Vox, and even Yahoo! 360, MySpace, and the like, which let you search by contact, by affiliation, by degree of friendship or by network.
The point is that you can’t rely on any single blog search engine alone. Sure, if you’re searching for relevant web content by keyword, there’s always Google. But with blogs, it’s mostly the conversations that you’re looking for and not just the content.