This game is too addictive. Dangerously so. If I were more left-leaning, I'd probably demand there be a law to protect us from this type of game. I'm really enjoying this game.
I have little to add on top of what other bloggers have already stated. Ogrebear, in particular, has a great series of tips in deckbuilding.
My only complaint with the game so far is that the only way of trading cards I've seen so far is a lousy chat channel. The last time I got into a collectible card game (Magic: the Gathering), I ended up with garbage bags full of commons and uncommons I couldn't unload (if you play mostly with other hardcore collectors, there's pretty much only card per booster pack that anyone would have an interest in). That's even worse in this game, since cards can be used in multiple decks, meaning I need fewer duplicates. Better tools for trading could help mitigate against this problem somewhat.
It would be nice if these virtual online card games would take a step beyond what physical collectible card games let us do already. For example, since the cards aren't physical, and just a bit in a database, it would be nice if there was a way to trade in unwanted cards for some minimal value (for example: deleting ~30 or so unwanted commons could get you a free booster pack; the actual number would have to be set so as to still encourage trading and what not). I'd be more inclined to buy MORE boosters with a system like that in place since I would know that even the common "crap" I get that I will never use and could never trade could still be transmuted into something of value.