I've used it several times, excellent system as you said!St. Louis gets no love from anyone, including people in Missouri. However Metrolink has very good service between downtown and Lambert St. Louis Airport. Using CityNerd's criteria:
Service from 8th and Pine starts at 4:13 a.m. everyday with trains every 20 minutes. The ride is a little long at 40 minutes, with the last train leaving downtown at 12:13 a.m. The trips from the airport downtown start at 4:56 a.m. and end at 12:56 a.m. The fare is $2.50.