Take the MTR to Sham Shui Po and take the exit to Apliu Street. The whole road is full with shops for GSM phones. Some of the phones have no warranty at all, some have only the british plug charger, others do have warranty (at least for HK). There is not much cheating going on there, on Nathan Rd you are very likely to be ripped of in most of the places.
For cameras the best shop that I know is New Sankyo Camera Ltd. on Mody road outside the Holiday Inn building. Tony, the owner has good knowledge and he has never been further than 50 HK$ away from the best price offered after extensive shopping around. I deal with him since 16 years.