i'm planning to visit shenzhen & HK around mid of may 2013.

Hi, to be frank, probably not a good travel season then.
For the former destination, it will be hot and humid. During that time, the average temperature will be about 26C and the highese temperature may reach 33-35C. So the summer clothes can be taken along, such as short-sleeved shirts and shorts. Due to the frequent rain, an umbrella or the rainy gear can be needed.

As for Hong Kong, it will turn from warm to hot at that time. And the average temperature will range from around 20C to 28C. At the same time, the drizzle happens less and it will be replaced by the rainstorm. So maybe you need take along a light overcoat and the summer clothes then.

Nice day,
Alisa