The Reinforcement pack adds some new ships worth having, and doesn’t cost much.
The Broken Alliance adds 8 new side-story missions to the (open) main campaign of the game, and some new ships as well.
Anabasis adds an entirely new game mode where the cylons chase your fleet while you try and protect the civilian ships, analogous to the TV show. It also adds some new munitions and some new missions to the main campaign.
The other three all add new, stand-alone campaigns as well as a few ships that can be used in any campaign/game mode (if researched).
I have the first three, and held off on the latter three since I haven’t even finished the main campaign yet.