I've been using Three's prepaid SIM for a little over a year. It's also got international roaming, a cheap international direct dialing (IDD) service, and you can add value using a credit card via their web site. I don't know how the price compares to the others, though.

(See http://three.com.hk/website/pagelet/...x.jsp?lang=eng -- it's the "DualBand" service)