A very convenient way to get to the centre of Guangzhou from Hong Kong Airport is to catch the direct bus from the airport, it will drop you at the China Marriott hotel and final destination the Guangzhou hotel next to the Pearl river. It costs about RMB 120 and you change coaches at the border. It takes about 3 hours, the same as metro/train/taxi combination into and out of Hong Kong City, but with less hassle.