Noah Smith tells us why he likes Michael Moore's films. In general, he likes them because they make people see the real life implications of economic and political policy. A lot of people don't like them for a similar reason. He particularly liked Sicko because a lot of the criticisms that it makes about the US healthcare system were made apparent, and people are trying to deal with some its flaws, e.g., the fee for service payment system.
Of course a lot of people don't like to be exposed to ideas that make them question their carefully groomed belief systems. There are several Internet sites devoted to trashing him. I found the comments following Noah's post interesting. He has a rather thoughtful audience for his blog.