I'm not sure how I feel yet. The campaign itself doesn't feel significantly different from Dark Crusade. I've only played one battle with the Sisters of Battle, so I won't say anything there. It was the tutorial. The changes to the Necrons seem minor. One bit I do like is that the power generators have little lights on them indicating how close they are to being used up. The new "unit" for Necron is something I'm undecided on. When you research the Nightbringer, you also get acess to a Deceiver option for your Necron Lord. The Deceiver has a very weak ranged attack, can turn an enemy to your side for a period of time, and an illusionary Monolith. It's nice, but to use it, you have to forgo the Nightbringer, so it feels more like a new ability than a new unit.
I played one battle with the Dark Eldar and they were hard to get used to. They are very different from any current race. They have a set and forget build system, which you may have seen in other games. Once a worker starts a building he can be reassigned, which also means you can't speed it up. Though it seems worker downtime can be used for the harvesting of souls or whatever the new Dark Eldar resource is called.