It's Not What You Know... is an excellent card, and can be considered viable for almost any deck type.
Firstly, this card is one of the select Resolution phase cards, which can adjust the outcomes of shootout hands after all the other shootout cards, bonuses, an effects have been used (Cheatin' Varmint, Point Blank, etc.). Basically these cards are the final surprise, and this surprise can REALLY sting! For 2 GR, you can lower an opponents legal hand 1 HR, or a Cheatin' hand 4 HR! No other card can lower an opponents hand rank so much and so absolutely. This is probably the best way right now to punish a Cheatin' hand.
This card is also useful in basically ANY deck. Regardless of faction, strategy, or tactics, you want this in your hand if you ever find yourself in a shootout. This is doubley useful in high-value decks such as Hex or Miracle decks.
The only downside would be the cost. 2 GR is a little pricey for a one-shot card, and it can be hard to keep 2 GR in reserves in a tense game. However, with a good production method, this should be easy to deal with.