I watched it on youtube in like 10 parts. I really enjoyed it. You aren't going to learn anything new if you are familiar with what went on back then. The interviews make it worth watching though IMO.
When I watched it I was expecting more, but it was still sort of enjoyable. Frost and his little performance.
I saw it here in Philly, with a Q&A with the writer/directors. I honestly wasn't that impressed. I didn't feel like it gave any kind of different perspective or any particularly new information that varies greatly from everything we already know about these bands/this scene. It features all the people from all the bands that you would expect to see in a film like this. I was hoping for a lot more. I think maybe the most impressive part was that they got some of these guys to actually trust and befriend them and let them tag along with a camera.
There's a torrent out there with about eight "Black Metal Documentaries" ranging from inane to funny to excellent, although not all of them are about BLACK Metal. IIRC "Once Upon a Time in Norway" is the most in-depth and least sensationalist one - and definitely better than "Until the Light Takes Us" which I also found rather meh. And "Anvil" is of course fucking brilliant!