If you play DC like I do, this is a LONG game. To start with, you have a choice of 7 'races' (Space Marines, Eldar, Imperial Guard, Tau, Necrons, Orks and Chaos) and each plays differently. The game overlays a very basic 'Risk'-style territory campaign map onto the skirmish game.
Pluses: a great look, interesting race differences (units, combat style, tactics), varied terrains with lots of inherent challenge, chance to buy units of 'Honor Guard' for each mission on the campaign map if you have completed other missions, giving you a running start
Challenge: aggressive (sometimes VERY aggressive) AI makes it just too hard to succeed on some maps; falls victim to the 'unit rush' style of play almost invariably, but some maps suggest that this is not inevitable, just a result of lazy planning/lack of imagination
Bad bits: although the system saves most of the 'hard' additions you make to conquered maps (bunkers, turrets etc), it starts you off with no units on conquered maps, whereas if you attack an AI-occupied map it already has units and often overwhelms you before you've even got going. This is unrealistic and soon becomes incredibly irritating, especially when you are playing a 'finesse' race (Tau, SM) against one which uses huge numbers of units to overwhelm (Eldar, IG especially). The unit cap on certain units (vehicles) seems to make little sense. The repetitive element of constantly defending conquered territories in the same way against the same enemy is tiring. And NEVER autoresolve - you'll pretty much always lose.
Overall: definitely worth the money, absolutely if you are a 40k fan, but with some unrealistic elements which could have been sorted out before release and which would improve gameplay considerably.