I saw it this weekend, and it had its moments. Thanks to the previous poster above who set out a link on some inaccuracies, inconsistencies, etc. in the movie since I found the counterarguments useful as well.
That said, I don't think that Moore's substantial artistic licence and editorializing detracts from what I thought was the main theme of the movie, that being that Americans seem to live in a much more violent society than other Western style democracies, and that they kill each other at a higher rate.
Did it deserve an Oscar? Probably not. Is it one person's "opinion piece"? Probably. I don't think he should be criticised for expression his opinion, but he can certaintly be criticised for using his position and "celebrity" as some form of justification of his point of view. Then again, a lot of people are guilty of that.