There's no restriction who buys these cards.

Services are similar among carriers. I use PCCW, others like People's. Main thing you need to decide is whether you're going to call local (HK) more, or international.

Both PCCW and Peoples have two main kinds of SIMs. One is very cheap local (as low as 5-6 HK cents per minute), but expensive international. The other is 25 HK cents per minute, but has reduced or free international "IDD" rates.