And it looks brilliant.
Jackson describes more of the production process in an interview he did with Mark Kermode. Apparently, they took archival footage that the British Museum had, and then digitally cleaned up both the image and the frame rate (including interpolating new frames to smooth it out), followed by colorizing it. They also used actors to make the footage into talkies, even going so far as trying to find the right accent for where the unit was from if they could. For the voiceovers, they used a series of interviews the BBC had done with WWI vets in the 1960s.
It looks pretty great. The reviews I've seen have all been pretty positive. It's only available in the UK so far (with Jackson arranging for a copy to be sent to every high school in that country), but I'm hoping it will make its way over here as well.