Going out of Hong Kong Airport during layover

After arriving by 10:45 a.m, I shall have a couple of hours before connecting a flight to Toronto at 5:10 p.m. same day. I should have enough time to exit through customs and go to Kowloon to do some shopping (I carry a HKID card). Is that OK? Is there anything I need to bear in mind in making such a trip outside the HK airport?