This week Magic The Gathering - Duels of the Planeswalkers for the PC was released via Steam. I had preordered it, so I got the first expansion for free. I played only a few games yet, at medium difficulty level, and the AI is playing okay. As you can't really build killer decks (only modify pre-constructed decks with cards you unlock by winning games), the AI even wins often enough to be a challenge.
Of course DotP is only "Magic Light", and has a couple of flaws, like the limited deckbuilding, and the Co-Op campaign not available online. But you can play against your Steam friends. And overall the game is good value for money, at just $10 or €9. As somebody on the Steam forums remarked, the game comes with a voucher for a deck of Magic cards from your local store plus one promo card (that would be paper cards, an ancient technology used in the previous millenium), which are worth more than the purchase price of DotP. So after you sell those to one of the bearded geeks in said local store, you might even be richer than before you bought the game.
I'm not sure I'll play a lot, I kind of burned out of Magic a decade ago. But given the price, I can only recommend Duels of the Planeswalkers.