Very addictive non-linear campaign that makes for a great expansion!
The Tau is one of the best races to play in the campaign due to their strength in range combat and strong vehicles. The necrons are fun when you managed to activate the dormant monolith. The new units for the other existing races doesn't add much bite except for the Chaos Space Marines whose ultimate hero upgrade may be something to behold. In the campaign, most of the stronghold missions are also impressively designed to vary themselves from the usual map clearing skirmishes.
Personally, I love the versatile campaign format which some fans may think otherwise due to the short-change in the story where Warhammer's universe is so rich of. But for most fans of the RTS genre, Dark Crusade is a great game for Warhammer novices to jump into without having to deal with the heavy backstory. A worthy expansion!
KingLim