Cannon Battle (6239) is a small set, larger then the impulse packs but smaller then eg. Loot Island. Still it is one of the best value packs considering the amount of bricks you receive. You get two minifigs: one Imperial Guard and one Pirate with some handweapons. It is the cannon that is one of the most expensive items in this expansion pack though, and next to all these things you also receive a small background wall.
My conclusion is that you get quite a bit for a measly 5,99 dollars, therefore this is a great expansion pack, one you need if you buy the fortress that only comes with one cannon!
Now rush to the Pirates forum to discuss this new 2009 Pirates set: 6239 Cannon Battle!