Ive played it for all of one day, but regardless...
Single player has a campaign, so far, really really good.
But that isnt what sells it. Its the Skirmish mode. Every map is procedurally generated, so you will never play the same map twice. Endless strategy and tactics, what with the 4 different unit types (that I know of) and their strengths and weaknesses.
Even in the Campaign, lets say youre on a mission and you up (very, very easy to do). When you restart the mission/map, its a completely different layout, so none of your moves in the prior session even apply.
Its quite hard and I love it.