From what I wrote on the blu-ray.com forum:
I got this disc from Amazon yesterday and watched about an hour of it. (The total time of the main feature is 90 minutes.) As documentaries go, it's pretty dry and very repetitive; it focuses entirely on global warming and paints a pretty dreary picture of the world's future. The PQ varies widely, ranging from old news footage to present-day shots. Most current footage looked pretty decent, but there's nothing that had the typical HD 'pop'. AQ was fair, but hey, it's just a spoken documentary! Overall, I have to say that I was a little disappointed with it. I don't think I'll want to watch it again, fascinating though the subject matter was.
I also said (in another post in the same thread) that I halfway expected it to be similar to Al Gore's An Inconvenient Truth, and while both films were about the same subject, Six Degrees was far less involving and entertaining.