It's several times stronger than steel, and a 9.5 on the mohs hardness scale, but only 40% of the density. Downside: brittle like all ceramics. Graphite fiber composites are probably already better for tensile purposes. It's waaaaaaay easier to synthesize than diamond. Synthetic gemstones of silicon carbide are circa 2 orders of magnitude cheaper for the same size vs synthetic diamond. With the right industrial process, and cheap enough energy, it might end up being as cheap and abundant as steel. The atoms in it are cheaper than the atoms in steel, but more energy is required to rearrange the atoms. So this depends on future energy prices.