HKIA has quite a good selection of electronics and gadgets, everything from laptops to mobile phones and everything in between. However, I find the prices much less in the city than at the airport, but depending on where you're from the prices may be reasonable.
If you're in transit, the Sound and Vision stores (both ends of Terminal 1 airside) have a good selection of items, the store near the lower numbered gates is larger. The Fortress stores are located outside the secure areas and will require you to clear immigration and customs to access them, as well as clearing security again to catch your flight.
I've bought quite a few things at the Sound and Vision stores, mainly because of how convenient the shop is rather than for the prices. Have a look around, there's tons of shopping at HKIA, but for the most part I think the shops are overpriced. The best thing to do there is eat, HKIA has some of the best airport restaurants in the world at fairly decent prices.