Check out his antics on LWN sometime. He is biased out the ass (at times outright being a lobbyist) and quite unpleasant to boot. Many of his "tactics" reek of bullying, such as insisting that people who he disagrees with reveal their personal details.
See this discussion for one relatively tame example of why people intensely dislike the guy: http://lwn.net/Articles/437650/
For extra fun, in this discussion he seems to be entertaining himself by attacking PJ's gender.
He's a patent lobbyist. Who exactly finances his operation is in part unclear (he seems to work for Microsoft on one project). He does not want to disclose his income sources. Some people feel that he's highly biased against Google etc.
FM is generally well informed, but may have a hidden agenda, you have to be awake when reading his opinions...
My main beef with his persona is that many news reporters call him an "expert" and publish his opinion as if were facts set in stone.
