The base campaigns are great for W40k fans. The protagonist from the first game makes a re-appearance. From a lore perspective, the game picks up post Cadian fall and introduces Guilliman and a certain Necron collector. The Necron campaign is also hilarious as it shows the constant bickering and scheming among court factions. The Tyranid campaign shows each faction's increasingly panicked response to the growing menace.
I play it for the music alone running in the background. It really sets the mood that the world is ending and there is all this sadness about how the ancients wrecked the world. The story is uncovered slowly by the reading of journal fragments (some of which are optional) and discussions between the characters. Conversations between characters are a highlight - everyone is included so even if you don't add them to your team they follow behind you in the movement mode - it is only when the computer switches to combat that your chosen team makes a difference. Mix and match different team members to overcome obstacles. My only criticism so far is the leveling system - the number of buildings and character point distribution could be streamlined better as it is initially confusing.