For me, the card was rather designed as a new and less broken version of Conceal that also works more safely. The idea of the card is just to set-up lethal by going all-in with your board, making you untargetable and letting your opponent do whatever with your board, then using the Copy effect to copy a valuable burst card like Eviscerate or Leeroy.
Basically, the card opens you a window to stop interacting with your opponent and go for a 2-turns kill. It counters Freeze/Control Mage's burst if played correctly, and gives you a safe turn in the end game where any aggro/midrange's topdeck is capable to crush you on the spot. One of the Rogues's biggest difficulties is to setup the perfect moment to go for the kill, particularly when the meta is very board-domination oriented, and a good fractions of your losses could have been a win with "just one more turn". That card brings them both ;-)
OK, people, I am searching for a DK to craft and Valeera is second on my list. Question: how to play her and what decks is she good in?
Just quoting a dude, whose comment I really liked.
She's a total kickass in wild mill. Other than that, she's too expensive, lowering arcane giants to 0 until T9 in miracle build is very hard.