The 28X is also a great option from CMU, too. It leaves from Morewood & Forbes, and its last stop is 5th and Morewood. You always get a seat on the way out (I've seen Pitt students in Oakland get turned down because it was already full from CMU students.), and if you fall asleep on the way back the bus driver will kick you out at your stop!
As long as CMU still offers a bus pass on your student ID, it's also a free service to get to the airport.