I have 3 complete sets, the original, the long rectangle and the worsted weight. They can be used for flat knitting AND round knitting. The long rectangle and the worsted weight set can be used for flat double-knitting as well as flat and "in-the-round" pieces.
Another user here, Joel, used the long rectangle to make an afghan out of several double-knitted panels. It was lovely.
My 8 year-old-daughter learned to knit on them, and she loves them. I have also recommended them in the past for folks who would have trouble keeping stitches on regular needles, like the elderly and those with arthritis.
There are videos on the www.provocraft.com
site to teach you how to use it.