Sticking to the usual names recommended by CSers here should get you reasonably reliable goods - Tin Cheung, DC Fever, Wing Shing etc. Went up recently for a tour of the shops there and also enquired about grey sets, most of them do not carry them anymore. Typically shops that sell predominantly grey sets have a high promoter-to-customer ratio, came across one where all of the men wore pink polos, price was lower than in Singapore but when pressed for an International warranty they try to throw sand in your eyes by insisting they provide a warranty. Just keep persisting in asking if there is an International warranty until they admit that they only provide store warranty.
Prices are not fantastic, generally just slightly cheaper than here. Camera bodies are cheap though, but as mentioned above the warranty is local even in the reputable stores, perhaps that explains the price discount. Of course if you buy a more expensive lens your savings are greater, but more and more HK prices are mirroring those in Singapore. In fact some accessories are more expensive, do your homework before going over.
If you do have the intention to purchase a lens though, and you know you don't sound like a local (speaking Cantonese doesn't immediately qualify you as a pro-bono Hong Kong native - their version of Cantonese has some variation in words used that can easily identify you as a foreigner), do check your item before you leave the store. The first lens they handed me had a visible speck of dust inside the front element - probably rejected by most of their local clientele, and they were hoping to foist it off on me. Just ask for another copy, and bring your camera to test the lens for sharpness, same process as here. The big reputable shops hold tight to the first price they quote, bargaining is tough, but you can try your luck.
At the end of your purchase, make sure you come away with a shop receipt bearing the date of purchase and signed by the shop salesman, as they will not stamp the warranty card, so NSC needs to see the receipt before they will honour the unfilled warranty card.