Ya Ya I know I'm a nerd... But seriously been playing this since October. One of the few games my gf will play and is totally addicted to as well. After a couple months I decided to buy the naxx expansion but otherwise haven't paid for any packs (totally free to play).
This is basically like Magic for dummies. Games last not much longer than 10 mins so ideal for quick plays. Unfortunately to play you need to create a battlenet account (this stopped me from playing a few months earlier). Statistically speaking the games plays very similar to heads up poker.