This is't really a clay, but two-component epoxy putty.
There are two brands of epoxy putty I know of. The first is Kneadatite, the brand found in the Games Workshops. The second is Milliput. I've only used the first, so I'll base my strong and weak points rating on Kneadatite alone.
I forgot to tell how you use it. Kneadatite comes in two-component strips (yellow/blue). You cut a piece of the strip and knead it until it becomes green and very sticky. You then sculpt it onto a small iron wire armature. It dries in about an hour and is completely hardened after 24 hours. This way you can work in layers; first the basic layer (e.g. an arm), then the big muscles (e.g. biceps, triceps), then a third layer with veins and details (e.g. bracers).