A great expansion that's just a little late.
The general mechanics and strategies of the game haven't changed dramatically. It is in essence, the same game. The biggest changes to the gameplay itself is...
a) The initial Tau units can stealth and fire, which can be a nuisance.
b) The Necrons don't need that many strategic points to kick you a**.
c) The Necron Lord's transformation is ironically more powerful than the Necron relic unit.
If this expansion were released about six months to a year ago, I would not be so harsh. But in all honesty Relic really outdid themselves with Company of Heroes. While they are two entirely difference games (WWII recreation vs. sweet sci-fi backdrop), you're left wondering what Dawn of War would have been like using the Company of Heroes engine and how much better of a game it would have been.
If you're a diahard fan of the Dawn of War series, definitely pick up this expansion. It is clearly the most polish of the three releases and unlike the Imperial Guard, the new races aren't crippled with fragile infantry (don't bother quoting "L2P", IG vs. Space Marines... who wins?).
For those who don't have any of the previous Dawn of War iterations, I would honestly pass this one up. In LAN and Multiplayer you can only play with the two new races. The fact you can play the others in the single player campaign is almost a slap in the face (yes, I own all three). For you, wait until this game hits the bargain bin or save your $30 towards another new release you've had your eyes on.