Just popping back to say that I took the Metro from SOF into town yesterday, and it's super-easy. Follow the blue arrows on the floor at SOF after you go through Customs. They'll take you outdoors, and you should look to your left where you'll see another building with a big "M" over the doorway. Once you enter, there's a machine at the bottom of the ramp on the left where you can buy a Metro ticket (I used a two lev note, and it gave me change). There's also a counter where you can buy a bottle of water or a small snack if you're absolutely dying.
Put your ticket into the turnstile and be sure to take it back before going through to the platform. There was a train sitting there when I went through, and it was clean and roomy. It took 15-20 minutes to get to the Sofia University (Kliment Ohridski) stop, where it was an easy walk to the Crystal Palace Hotel. (The underpasses can be a bit confusing, but just be patient and turn around and try again if you end up across the street from where you want to be.)