True. The cabbie will probably charge you a little extra for luggage, and also the toll on the motorway. But if there are three of you, and it's late, then a door-to-door service is worth paying for.
Yes - still €9 (I was there a few weeks ago). The thing to remember with the Athens Metro is that the Airport services only run every half an hour, and I think stop around midnight. And on your return journey you'll probably find yourself at a Metro station (eg: Monastiraki) where there are...