Blizzard hit a milestone recently with the free-to-play digital card game Hearthstone: Heroes of Warcraft breaking 20 million registered players. For scale, that's double the number of players WoW had at its peak during Wrath of the Lich King in 2010. The most amazing thing about that is Hearthstone has yet to release on all intended platforms: Android tablet, Android phone, and iOS phone versions are still forthcoming.
All in all, the registration numbers indicate that there is very widespread interest in Hearthstone indeed, and for good reason. It's a ton of fun to play!