For players just joining the series,
Dawn of War II introduced a completely new approach to the typical RTS experience. There's no base building, resource gathering or, well, anything that would identify it as a traditional RTS. But that's what made it such a great experience. Instead of worrying about the front office stuff, you're able to jump right into the fray, commanding a small platoon of troops for the glory of the Empire.
Warhammer 40, 000: Dawn of War II - Chaos Rising is identical to Dawn of War II. You can choose to bring your army from the core game into the expansion, giving you another opportunity to level your crew and equip them with new items. If this is your first foray into the Dawn of War II universe, you're given a sturdy level 17 group the minute you touch down. The pre-made group is great, but removes some of the game's appeal. Leveling and looting is a major part of the gameplay, so just jumping in with an already-leveled group cheapens the experience a bit. Combat is fun, looting is great, but it's the bond you create with your squad that makes the game worthwhile.
Regardless how you enter the battlefield, Chaos Rising offers a lot to do. The adventure isn't nearly as long as the main game, but brevity has its benefits. There's more variety in the types of missions you encounter, and the ones you repeat are spaced apart just enough that they don't drag.
The limitations of going with just the expansion show up in multiplayer. Nothing changes with Chaos Rising other than the addition of a few new maps, a new game type and new hero units. However, the expansion limits you to only playing as Chaos rather than the original four sides. If you like Chaos it isn't a big limitation, but the new hero units for other sides are really nice. Besides, no matter how much you like Chaos, you're going to want to switch at some point.