I am halfway up rank 2, from rank 4, using Mill Rogue. I actually have not lost to a Pirate Warrior yet, and I just beat an Aggro Druid. Shadowblade is SO good against aggro because you get to hit twice but you can keep your health up a bit. I used Envenom Blade in Mill Rogue after the last set, and while it was powerful at controlling the board I usually fell into burst range from taking so many hits. Just stuff like being able to Backstab into Shadowblade a Berserker is game-changing.
Valeera DK is on another level in Mill Rogue. Once you get her into play, every Sludge Belcher is 2. Every Antique Healbot is 2. Every Coldlight Oracle is 2. I even had a match where I played Antique Healbot, my opponent couldn't kill it behind the Sludge Belcher, and so on my next turn I was able to N'Zoth and then double Shadowstep, with the second Shadowstep being a free one from Valeera DK - that's another 8 health the DK hero power gave to me. And since the deck runs double Vanish, you're never going to get punished for playing the DK. On multiple games I have went:
1) Valeera DK (end turn).
2) Opponent plays a bunch of stuff.
3) Coldlight, Free Coldlight, Prep, Vanish.
So now that free DK I got from the DK hero power is a real and permanent one in my hand, I just wiped the board, and I on my next turn I will be able to cast 3 Coldlights because I get ANOTHER free one. WOW. The Valeera DK just improves the deck's consistency, fluidity, and number of answers to the 10th degree. I'm always looking for her in any matchup. Just a fantastic card, and the first time I've crafted a cardly early on in the expansion without regretting it. Even if Mill Rogue ends up mediocre in the end, she's just a lot of fun and very interesting to play.
Now why couldn't all of the DK cards be as fun and interesting as Rexxar and Valeera?
Meta is so greedy right now. Currently trying to build a deck to punish everyone for holding 10 cards every turn.
Play Jade Druid.
See nothing but Priests with Skulking Geist.
Play Mill Rogue.
See nothing but aggro and midrange.