i have always found metagross to be a powerful threat, before gen 6 its only real threat was fire types that could outrun it (almost all of them)
however as of gen 6 it was nerfed a bit, aegislash with shadow sneak could stop a metagross with ease due to steel now being neutral to ghost, dark also became an issue with greninja
my XY build for metagross focused mainly on taking hits while boosting with agility and power-up-punch, then unleashing massive damage at a high speed
its moves are meteor mash, earthquake, power-up-punch, and agility
before ORAS i used the iron plate as its item, that along with STAB doubled meteor mashes already great power
now, i do realize that this move set causes an issue with the whirlwind skarmory build, but i use charizard as its counter
EVs are 36/252/18/0/108/96, adamant nature
it was able to survive 2 EQs from my friends garchomp (neither my metagross or her garchomp mega evolved) that was IV bread, that along with that the only thing that i seem to encounter that can OHKO it [psychically] are those annoying revenge killer weaviles
with the addition of its mega evolution and the great boost of speed i normally only need 1 agility to outrun my oppenets, sometimes i dont even need it (im considering getting rid of agility, but it still needs more testing)
tough claws along with the boosted attack give its meteor mash more of a power boost than the iron plate would; it also helps EQ, so the mega stone is worth it
altogether i think that mega metagross is a great addition to OU, and i hope to see a comback in its popularity