It's great that there are newly added stops to Hongdae and our town, Gongdeok.
The airport subway is pretty convenient, but takes a bit long. Be prepared to bring something to entertain yourself with.
The pros of riding the subway over an "airport bus" is that it's much cheaper. Less than 4000won one way. But again, it takes a bit longer than the direct bus. The direct buses to the airport takes around 50 minutes(more or less), but this depends on the driver and where you are.
Either way, getting around to and from the airport is not so bad and you have many options.. direct bus, regular subway(airport subway), or the faster train (which is not really that much faster..shhhhh don't tell them I told you that..). And of course, there's taxi - which is not an option unless you have money to spend. It takes 60,000won and up from Gongdeok. Don't take one unless you're a group of 4 or more and have many many bags ;)
They have been advertising the "faster airport train" heavily (that runs to and from Seoul station) - but it's only 15 minutes or so faster and runs at odd times(every 30 minutes?) - so I never caught it. I think it's a waste of money on their part - I would only take one if it was 30 minutes faster. It's too bad that's not working well. They have slashed the price pretty low now.
If you have some time to spare and don't have many bags to lug around, I would recommend the regular airport subway.