The creation of a chunk of metallic hydrogen just a few days ago should set pulses racing amongst sci-fi writers and space geeks. If it turns out that metallic hydrogen remains metastable once they start backing off the pressure, then the door might open, just a crack, for a rocket fuel to enable SSTO, Single Stage to Orbit, rockets.
It's actually better with a little sci-fi materials tech, because the more realistic options involve a diluted form of the fuel. Pure metallic hydrogen would need materials capable of withstanding 7000k temperatures. We don't have any yet. So diluting the metallic hydrogen with water or liquid hydrogen is required to bring temperatures down to something more manageable, and that, sadly reduces the specific impulse. Still way better than liquid hydrogen/oxygen fuels, though. But hey, what an incentive for someone to take a look at high temperature materials research.
There's more detail, with the maths, about metallic hydrogen as a rocket fuel here: http://bit.ly/2kiJb5r
If nothing else it gives us a label to use in medium-future stories that can make hopping into a ship and flying off to the moon without having to scatter giant fuel tanks all over the globe, a little more realistic.
And I haven't even mentioned the sci-fi transportation applications for room temperature superconducting metallic hydrogen, giving us, amongst other things, super fast maglev trains. Fwoar!
I'll be watching the metallic hydrogen story with great interest.