I've played a whole lot of games with the new C
A, and with four completely different armies (one I just posted in the army list forum).
They have a whole lot to offer, but I've found a few strengths that are undeniably powerful.
1) Hybrid Ravenwing-Deathwing army
With the ability to make a 12" scout move, then deep strike terminators on the first turn, then move forward and charge into combat with bikes OR turbo-boost safely into a more strategic position, this army has a lot of versatility, and a whole lot of potential for a first turn close combat charge. Very overpowered against an average opponent.
2) All Ravenwing army
It's amazing how many landspeeders you can cram into an army like this, especially if you utilize the Ravenwing Attack Squadron to get extra Tornadoes. The long range firepower available to an army like this is pretty good with all the assault cannons, multi-meltas and plasma guns it can field (and fire on the move). It can also assault on the first turn.
3) Devastator-based Army
Three Devastator squads become six, since you can split them up, and since the Plasma Cannon is now cheaper than it was in C:SM, it's definitely worth taking lots of them (and lascannons as well). Take four heavy weapons per Dev.Squad and split the squads. This gives you the ability to maximize the effectiveness of the weapons since you only have to fire off two at a time before picking what the next two will target. The remainder of the force can be Tactical Squads with Las-Plasma and powerfists for counter-charging capability.
There are lots of other variations of the Dark Angels too... too many to type out right now