I had mixed feelings about the concept of The Plan before I saw it: Olmos spoke a lot about The Plan at a panel at '09 Wizard World Convention in Philadelphia. He was very excited about the project and led us to believe that we would be presented an intimate look at an extended Cylon plot that ran concurrently with the series as a whole.
He did not deliver.
First, the movie is at least 40% Archive Footage (taken from Season 1 and Season 2). The scenes are woven in as helpful "reminders". These "reminders" help connect events from the show to their extended explanations in "The Plan". Truthfully, the archive footage is abrupt and unnecessary. Fans would easily be able to connect "The Plan"'s events.
The story of this movie lacks the provocative and thought-inspiring nature that BSG is so respected for. The writing is poor and the events play out in rather unspectacular way.
The only thing that saves this movie is the strong performances by Dean Cain, Tricia Helfer, and Callum Keith Rennie. Though the writing is not up to normal standards, these actors' mastery over their characters is readily apparent in this flimsy companion piece.
Overall, watch "The Plan", it's not very long and has a few interesting moments (spread far out in the film). More importantly, if you're interested in Brother Cavil, this is a great way to get some perspective on the bitter Cylon "leader." You need to disconnect your brain a little bit at some points (like seeing 7 Cylons hiding in plain sight on Galactica), and if you can, you can enjoy the bits of BSG-goodness hidden in the film.