Besides likely being a room-temperature superconductor (at ridiculous pressures, like 500 GPa), it's postulated to be metastable -- like diamond, you could create it at pressure, and it might stay a solid metal at STP conditions. It's postulated to be made of atomic hydrogen -- lone H atoms, without the molecular bonds of H_2. The recombination energy H + H -> H_2 suggests [1] it's the most energy-dense chemical fuel that exists, with 20 times the specific energy of {H2 + O2}. It could allow [1] rocket engines with I_sp of 1,700 seconds -- four times higher than LH2/LOX. It's thought to be the main phase of hydrogen inside the planet Jupiter [2] and responsible for its dynamo [3] (but as an ordinary conductor, not a superconductor). It's also speculatively a structural material, one that's less dense than water [4].
It might have been created in a lab, in 2011 [4], but it's not clear.
[0] https://en.wikipedia.org/wiki/Metallic_hydrogen
[1] http://www.nasa.gov/pdf/637123main_Silvera_Presentation.pdf
[2] https://en.wikipedia.org/wiki/Jupiter#Internal_structure
[3] https://en.wikipedia.org/wiki/Magnetosphere_of_Jupiter
[4] http://www.nature.com/news/metallic-hydrogen-hard-pressed-1....