Application prospects of MgB$_2$ in view of its basic properties
  Seven years after the discovery of superconductivity in magnesium diboride, the fundamental superconducting properties of this compound are well known and the peculiar current transport in polycrystalline materials is essentially understood. Based on this knowledge the ultimate performance of wires or tapes at high magnetic fields will be predicted and compared to state-of-the-art materials and to other superconductors. The key parameter for high field applications is the upper critical field, which can be strongly enhanced by impurity scattering. This fundamental property might be further optimized in bulk materials, since higher values were reported for thin films. The MgB$_2$ grains are usually very small, if prepared by the in-situ technique. The resulting high density of grain boundaries leads to strong pinning, close to the theoretical limit. On the other hand, the connectivity between the grains is still rather poor and strongly reduces the achievable critical currents, thus leaving room for further improvements. 
M. Eisterer and H. W. Weber