Coming from other FFG games, I'm looking at picking up some of the intro decks for casual games with other gamer friends.

One question for more experienced players, are these decks balanced against each other, or are there clear good and bad decks such that one deck would always win or lose between equal players?

A given deck usually isn't going to go 5-5, 6-4, or 4-6 against the other seven, if that's what you mean by balanced. With the wildly varying player skill when hopping between factions and general skill requirements, you shouldn't assume your players will be equal, even if you're all well versed in card games. For example, Night's Watch will probably go closer to 3-7 against Greyjoy at first due to what GJ is capable of along with different skill caps for the decks.

That said, when I looked at the decklists nothing jumped out at me as being abusive. FFG did a decent job of either reducing or omitting cards from the intro decks which might put one faction at an obvious advantage. If you're only in the market for a few decks, I think the groups FFG previewed together provide a good purchase strategy. Stark, Lannister, Baratheon, and Greyjoy vs Night's Watch, Targaryen, Martell, and Tyrell. If you're not looking to buy everything, I believe the decks will offer a nice variety within the groups and probably offer the most balanced experience.

Breaking the factions down into either aggro or control types will generally go like this: Stark, GJ, Targ, and Tyrell are more aggressive. NW, Martell, Lanni, and Bara are more control themed.