A friend was kind enough to let me have a Hearthstone beta key so I've been playing this yesterday and today.
It's a very fun and original take on a collectible card game. It has a lot of tactics to it and I'm really enjoying it.
The game works by offering a balance between modes : you can either play with constructed decks that you make from your collection that reward gold or you can play an arena mode which costs you gold but builds your collection.
The arena is probably the playstyle that most players will prefer, with constructed being seen to some extent as a grind you have to do to pay for it. That said, playing constructed games is hardly monotonous, it just doesn't have the rewards of the other mode. It costs 150 gold to enter but rewards you with gold, cards and crafting dust proportionate to how successful your arena run is. If your record is better than 6-3 you'll usually cover the entrance fee with change, plus a pack of cards and maybe some crafting dust.
Setting aside the rewards both modes are just about evenly fun in my opinion. In crafted you have the process of creating and designing your own deck and of course the more cards you collect the more choice you have, to some extent the more power. In arena it's nice because the card choosing is pretty fun then you play everyone on a level playing field, determined by luck and skill, not the size of one's collection.
I started out doing the tutorial then some of the practice games against the AI. After a bit I decided I might as well play people and managed to go up several rankings even though I hadn't worked out how to unlock deck-building and was just using the starter decks. (You have to unlock every character class to construct decks and play arena games). There's a bunch of rewards for doing various quests and I managed to get to 450 gold from doing that. Then I went into the arena. You get a free go, the gold from newbie quests covers and 3 goes, and some good runs paid for some more. My best run was 8-3, most of my runs were 2-3 or 4-3. I'm pretty average starting out, by the relatively high standards of arena.
It's a ton of fun because there's so much to think about, without it being in any way hard to start. I don't think Blizzard have ever been so successful at achieving a game that is easy to play, hard to master.