This movie is an excellent addition to the series. In fact it only covers the events of the first 2 seasons, but now seen from the cylon perspective. It must be said though that it is quite mandatory that you saw at least those 2 seasons, otherwise you won't have a clue what it is all about.
Now the spoilers :
The movie is a combination of 2 stories, 2 perspectives, and a whole range of subplots as we've come to expect and love in the Battlestar series. The first perspective is from a brother Cavil model (the priest), on board of Galactica who runs the anti-human movement. The other perspective is from another brother Cavil, on the planet Caprica, where he infiltrates the resistance. Both brother Cavils are in essence exactly the same, but it are their different experiences that set them apart. Initially they both hate all humans (and act very much like spoiled children), but gradually things happen that make it change. One becomes better, the other becomes worse.
It is all very subtle, and only in the final conversation is everything said in more detail (though still subtle). However the full motivation behind the cylon attack on the colonies, and their sudden change of heart later (proposing the truce), is all there - some other comments here unfortunately fail to spot it and bash the whole movie as a result. The skinjobs attacked out of frustration with their limited human form and the obvious lack of acceptance as human, which caused them to hate their creators. "As long as a human lives there can be no place for cylons". Coupled with that they are also jealous of the real humans, because the cylons can never be anything else than machines. There are more reasons, but like I said it is all very subtle. It helps to think of the cylons as children without much life experience. The change of heart comes from the experiences of the individual skinjobs placed among the humans. To put it cheesy : they learn what it means to love, and that conquers all hatred. E.g. Boomer could not kill Adama because she loved him (which seemed like a plot hole at the time : if you wanted to kill Adama, just put the second bullet in his head instead of his chest. This plot hole is now "filled"). Even the stubborn Cavil who is left on the planet learns the meaning of it.
So all in all, a very good movie, that should be watched together with the series.