Playable factions: Imperial Guard, Space Marines, Chaos Space Marines, Eldar, and Orks.
Completed the game on Normal difficulty (Disorder Campaign).
I completed the disorder campaign with Chaos! Excellent last mission - the Necrons are introduced, slowly walking towards the base. Their Monoliths were invincible, at least until the Chaos Sorcerer possessed the weapons of the almighty TITAN. An then the Necrons had to die.
Completed the game on Normal difficulty (Order Campaign).
Just completed the Order Campaign while talking to my father and not really paying attention ... It wasn't hard at all. The trick to starting usage of the Imperial Guard is in my opinion: replace all your 'T-shirt'- and 'laser pointer'-equipped footsoldiers with Kasrkins. Fully-upgraded Kasrkins are actually quite tough. Hellhounds are also cool. For my lazy style of playing, Sentinels are too easily killed ... they require too much micro-management.