Depending on where you are in Andalucia, public transport is either great or terrible. Along the Costa del Sol, your best bet is the ALSA buses. Inland near Sevilla and Cordoba, the AVE high speed train is very efficient. To go off the beaten track I'd definitely recommend hiring a car.
If you're approaching from the Spanish border by car one tip is to set-off early, as during the summer-season you can be waiting quite a while before clearing into Gib due to the locals and tourists all trying to get in/out.
