The CSL roaming SIM works great. The HK$280 card gives HK$250 worth of value. No recharge needed while card has value. I received a call while in Guangzhou from Italy with no problems at all. (Ok, this shouldn't be a big deal, but it is for this North American.)

HK$99 recharge extends card expiry for 3 months; HK$200 extends for 6 months.

Calling from locations outside HK is somewhat involved, requiring prepending several numbers to the number called and then getting a call back.

Also, the card's value isn't shared between voice and SMS messaging. You explicitly have to transfer value from your voice "pool" to an SMS pool.

Don't expect to get a CSL GSM SIM or recharge card from the airport 7/11 shop or many of the other advertised outlets unless you speak Cantonese. A good source is the 1010 Centre in Central, at the corner of D'Aguilar and Stanley Streets.
Rich