In July of 2009, some people held a forum in Beijing to discuss the effects of the plastic bag ban's first year. Here's a link to the transcript of that forum discussion.
China's Bag Ban, One Year Later
Sounds like in spite of spotty enforcement, the ban had been quite successful at reducing plastic waste.
"Our survey, conducted over more than a year, found that the number of plastic bags in garbage has reduced by over 10%. Beijing produces over 20,000 tonnes of garbage a day, with 10% -- about 2,000 tonnes of that – being plastic bags. A 10% reduction means 200 tonnes less garbage."
I'm not saying that I advocate a complete ban on plastic bags. But I do think that a tax on plastic bags--or requiring stores to charge for them--makes good economic and environmental sense. Even five cents a bag would be enough to motivate a lot of people to remember those reusable bags they already have sitting at home.