I'm nervously waiting until Thursday to get two tickets, it's going to be touch and go whether there's going to be enough left
Yes we went but didn't take the train back to Brighton. As there is very little car parking at the ground the options are Park & Ride, Coach or Train. The Park and Ride is a bit of a nightmare as there are long queues waiting for a bus although I believe they do move quite quickly. The car parks aren't near to Brighton town centre though.
The supporters coaches are probably the best bet as they park at the ground and get away quickly, but no chance of a beer after the game.
The train option looked to be horrendous. The station is small and the access is ok but the sheer number of people clog the footbridge to the platforms (think the number of people going over the railway line at St Mary's and double it). As you say, everyone is travelling back into Brighton so the amount of people isn't divided equally between the two platforms. That's all before you get to a train, but obviously that was before they got to the Premier so hopefully the number of carriages will have been increased. If you are not driving, it's the only way of getting into Brighton for a beer. There is a pub though in Falmer village, it's supposed to be home fans only but if you don't have any colours on who's to know? There is a footbridge over the A27 to Falmer which comes out in front of the pub The Swan Inn. Have a look at Google maps.