Excellent.
I bought 2x10kg (25 lbs) to put in a kitchen cabinet for organisation, looks, and easier handling of the weight.
For how I use them, they're perfect. I only have to handle a 12kg bag once to fill them up.
The materials are good quality and flexible, but I think they're not meant to be carried around outside the house—more for home use.
Where I have them (out of the sun), they don't get greasy or stain the cover.
They come with a black synthetic cover. I recommend not scraping them—they're not made of wood or metal—and to use a plastic or wooden spoon.
Very sturdy, but treat them with a bit of care.