This is a really strange and interesting material. It has featured in earlier posts. We discuss how the observed properties of both the "metallic" phase and the "insulating" phase are quite unusual and don't seem to fit into any "standard" picture [Fermi liquid, Luttinger liquid, quantum critical, ...]. We then propose the simplest possible lattice model Hamiltonian that might capture its properties. This is worthy of further study.
We thank Nigel Hussey for getting us interested in this fascinating material. He has a forthcoming PRL about the unconventional (possibly triplet) superconductivity.