Apparently we weren't the only ones waiting for Hearthstone to come out on the iPad. According to Business Insider, Hearthstone is the number one app in 34 countries, and the number one game in 36. This includes the United States.
Magic: The Gathering and other CCGs also have a large support base on the platform, so it isn't really surprising that Hearthstone would be popular amongst users of the tablet. What is surprising is that it rocketed up the charts so quickly. It is estimated that the game could pull in $30 million in the first year from in-app card sales, which would make this foray into the tablet space quite worthwhile for Blizzard.