http://www.wizards.com/Magic/Magazine/A ... daily/sf/1
In Brief: Each player opens a booster and adds lands, making a 30-card, five-color deck to battle with.
Rules Rundown: Each player needs one booster pack and three of each basic land. It doesn't matter who owns each booster pack; they're not going to be mixed, so everyone will be able to take their own cards at the end.
Open your pack, but—and this is important—don't look at it! Half the fun of Pack Wars is not having any idea what you might pull off the top. Add the 15 lands to your booster pack and shuffle thoroughly. You and your opponent now each have a 30-card deck that's ready to go.
Many groups disallow mulligans during Pack Wars, again because of the fun to be had in not knowing what might come out of your deck. But not being able to play spells is no fun at all, so there's still reason to allow mulligans. Decide with your opponent whether you're allowing mulligans before looking at your opening hands.
After that, it's normal Magic. Pack Wars is usually played with two players, because it's generally a pick-up game, but there's nothing saying it couldn't be a quick multiplayer format if there are more packs that need opening.
There's no rule that says how many games to play, but if I'm playing with one other person, I usually make it a best-of-three match.
Pros: You can play Pack Wars anytime you've got at least one other player, with one unopened booster pack and three of each basic land for each player. This makes it more or less the perfect format when you have packs you want to open and are looking to play a quick few games of Magic. It's better for this than most Limited formats because of the small number of packs and quick game time.
The fact that your deck is random and unknown is most of the fun of Pack Wars. It's exciting to draw that perfect card you didn't even know was in your deck! Pack Wars can also get you playing with cards or combinations of cards that you wouldn't ordinarily use, and seeing them in action can really change your perspective.
Cons: With five-color decks assembled randomly, mana problems are pretty frequent. You've just got to accept them in the spirit of random fun that inspired the format. If that's going to frustrate you, Pack Wars may not be your game.
And, of course, like any Limited format, you've got to open packs to play. But if you're opening packs anyway...