They are to be avoided at all costs.
When I say plastic bags, I mean the $1 store plastic bags with zippers or perhaps a plastic bag in the form of a duffle bag. In a customs line at the airport it never ends well.
This has now happened to me 3 times, watching the spectacle unravel in front of you.
I will admit, I smiled as the I watched the Chinese customs official pull out 3 bottles of liquor (carefully wrapped in tube socks), three bottles of assorted drinks, scissors and a brand new Zippo lighter still in the packaging. However, the ensuing loud voiced, arms waving argument between the man and his family with the 3 customs guards just went on and on. I politely tapped a customs official on the shoulder and indicated that it would be great to be passed through using my hands and a smile as I do not speak Mandarin.
I would have rather watched it from a line over.