I got 2 beta keys in my email this week (one of them probably by accident).
First - damn this game is addictive! It has some complexity like magic but the games are quick and varied. Each hero has their own special cards which means they all play differently and share a lot of strengths and weaknesses from their respective classes in WoW. On top of that, each class has a unique ability that's always available and costs 2 mana. Mage is fireball, warrior adds 2 armor, priest is a heal, and so on.
If you can get over the hurdle of dropping some money initially to buy some packs then you'll have a really fun time. It feels very much at though the original WoW devs are on it. It's well tuned, a lot of fun, and it's not a simple x+1>x game that WoW is now.