Who needs credibility anyway? I saw "Fahrenheit 9/11" last night. Of course it was biased, but it's not a news broadcast. That's like harping about the scientific inaccuracies in an NBC made-for-TV miniseries. I imagine that if there is anyone on the fence, this movie will push them toward the left, which is exactly what Michael Moore intended to do.
Nothing really new with regards to the movie, especially if you've read
Dude, Where's My Country which also focuses on the Bush-Saudi family ties. Not too hard to find
other background information if you're willing to do a little Googlework. In the end, I think the movie doesn't do much to add fuel to an already heated debate, but I think it will encourage more people to participate in the debate, which is always a good thing.