Cernit Metallic is a premium modeling clay produced in Belgium by The Clay and Paint Factory. The Metallic line of Cernit polymer clay is based on the Translucent line, and tends to be soft when very fresh.
The 250g and 500g bars are a good savings over buying individual 56g bars - you get almost 4 1/2 bars worth of clay for much less!
A great overview of Cernit Metallic polymer clay can be found here - Cernit Metallic Review.
For a comparison of the various brands of metallic clay and some tips on how to use it see Metallic Polymer Clay – A brand comparison.