The BSG series are scripted as they go along, but they put a lot of effort into script choices and plot lines, character development and all the other things needed to make it a riveting TV show.
BSG in my opinion is undoubtedly the best Sci-Fi drama I've watched, and I've watched plenty of Sci-Fi shows almost religiously. With BSG even the editors themselves have said that at times the audience has to "buy" what they're seeing (believe it could happen, believe it makes sense in the wider background of the previous episodes), but there are plenty of treats if you watch it through from start to finish.
Its not one of those shows that you can really just pick up half way through and get a lot out of it. You have to watch from the beginning to understand the direction each episode takes, there is a lot of references to history in the previous episodes/series' which isn't simply covered over and over. Rather, the show is geared towards those who watch it regularly and know about what has happened in the past in the show, instead of a program which you can pick up, understand and enjoy watching any single episode on its own.
A lot of the impact and key moments in the show depend somewhat on the viewer understanding the history of the characters and events, and in that sense it develops very well from episode to episode despite the way in which they move the show forward. If you listen to the podcasts of Ron Moore for each episode, accessable through the main site
http://www.scifi.com/battlestar, then you get a very interesting commentary on how the editors and entire development team work through the process of deciding how to take things forward and how the original script and plot of things often changes even in post-production to make the show more enjoyable, believable and interesting.