First of all let me get one thing straight, this is in no way an original Battlestar Galatica story. Razor had a story that centered around a new character and told a somewhat self-contained story. This is almost the complete opposite; no relevant new characters, no truly new plot lines. This film is essentially a side story of the original two seasons. It deals with the attack on the colonies and the subsequent invasion and pursuit of Galactica from the point of view of the Cylons; not just the original 8 models but also the final five.
The main character is actually Brother Cavil, in two different forms one on Caprica and one on-board Galatica. We see that Cavil plays a crucial role in orchestrating the Cylon events on the Galactica, while his Caprica counterpart takes the role of a distant observer to the struggling band of survivors on Caprica.
What I really liked about this movie is that it kind of ties everything together. It retreads the old plot lines of the first two seasons, but knowing the revelations to take place it puts everything in a different light. It also explores the intense cruelty and duality of Cavil. Additionally, some of the plot lines that were never quite clear, begin to make sense. (How did Leobin know so much about Kara? Where Valeri's actions intentional?) There are three issue that I can foresee someone having with this. The first being, that some might feel it being a retcon of the first two seasons to connects the events of the fourth. While this might be somewhat true, I don't think it takes anything away from them, and if you liked the fourth season it adds something significant to it. The second issue is that it is a retread. We already know exactly what happens to all these characters. But I don't think that the compelling part of this is what happens so much as why it happens, and what feelings and emotions are fueling the Cylons. The third is that there are basically no space battles to be seen here. There is a great opening sequence where we get a much more in depth look at the destruction of the colonies, but for a show called Battlestar Galatica we might expect some space jockey action.
I am not a prude but I must say that after seeing the unrated DVD, there is a bit of gratuitous nudity. Not that I had a problem with it, so to say, but it felt a little out of place, like it was trying to be a little shocking. Maybe it was just trying to give us a taste of what the show would have been like on HBO, i dunno just felt a little tacked on.
Overall this is a great companion piece to the series, does not feel entirely necessary but at the same time doesn't feel like it didn't need to be in the first place.