I returned from my holiday trip this weekend to my roommate informing me that DW Unholy is not only a viable spec but is also a lot of fun. I dabbled in Unholy ages ago, but since 4.0 I was sitting on a Blood spec I pretty much never used because that's what happens when your roommate is addicted to tanking. He nudged me to give it another try, and in between my struggles with Yellow Dot Syndrome I ran a couple Heroics with it.
My first impression of DW Unholy, post-Cataclysm, is that it is fast. The Runic Corruption talent speeds up the notably slowed-down rune refresh rate, and combined with Reaping you almost always have some resources to use. Festering Strike becomes part of your regular rotation, so you don't have to worry too much about refreshing your diseases. Wrath Unholy was a strict rotation spec, whereas Frost at the time was notable for the procs. Unholy has procs now too, most significantly Dark Transformation, which turns your ghoul into a bigger, nastier monster, and Sudden Doom, which gives you a free Death Coil.
Initially I was a little disappointed to see the different specs lose their flavor - it's like if DW Arms was suddenly a viable Warrior spec - but I really enjoyed Unholy for the few hours I got to play it. Once I've had a little longer to experiment with it I'll have more to say beyond my first impressions.