Considering how reluctant most people are to leave negative feedback, for fear of retaliation, I generally take negs seriously. Any feedback rating below about 99%, I take the time to look up in depth(at max page length). The odd complaint, I easily ignore - you can't please everyone. But when I see a pattern of complaints about the same type of thing, I avoid that seller.
Sometimes, you'll read buyer after buyer raving about fast shipping on great items, etc. but also a fair minority complaining about seriously late shipping, no answer to emails or calls, and items never arriving at all. Is the seller great or terrible? I figure he's just sloppy, hit or miss, maybe overextended. So, the question I gotta ask myself is, Am I feeling lucky today?