I visited Krakow in April 2012 together with my family (5 persons including me). We used Krakow Airport Express shuttle service both from the airport to the city centre of Krakow and back to the airport. We also took the tours to Auschwitz-Birkenau and Wieliczka Salt Mine that Krakow Airport Express where arranging.
They were always in time, had great prices, drove safely, spoke great English and were very friendly. I will definitely use Krakow Airport Express next time I get to Krakow.