The China Unicom SIM card you are talking about is kind of a PAYG/post-paid hybrid. You're basically prepaying for a bundled voice/data plan that gets deducted monthly at a fixed price, e.g. 240 minutes + 300MB data.

It's not good for tourists but has some advantages for those who visit China on a regular basis. The main one is being able to call in to "freeze" service, although reactivating is a bit of a pain and requires a visit to a China Unicom store.

China Mobile still seems like a better deal and I have been pretty disappointed with CU's 3G data service. Very sketchy in the Guangdong area.