I feel highly out-classed with my limited knowledge of the metagame, but I have been a Missy fan for quite a while now. After falling in love with the ghostly vixen when I made a mono-ghost team in honor of Halloween, She has not stopped impressing me. I typically run the sub-plot with max spd and sp.a, and it has brought on a number of late game sweeps. While the CM set provides more defensive staying power, I typically find the fast boost helps punch more holes in the teams of opponents over the course of her short lifespan than the CM investment is worth. I have even gone so far as to attempt swapping her in for the Gengar I used to run in OU. Slightly less speed traded to the ability to boost that special attack is worth it in my opinion 8 times out of 10.
Also, this Missy set, brought me my first ever 6-0 sweep. So I think that holds a special place in my heart and may create a bit of a bias.