Play Arts are 1/7 - 1/8 scale, though some of them might be a bit bigger or smaller. Basically they're a pain for me, because I like all those figs but don't want to get stuff that's not in scale with my Figuarts/Figma (1/12-1/13 I think).
On the other hand if you're more of a video game character fan, Play Arts Kai works out. You get Master Chief, Noble Six, Adam Jensen, Solid Snake, Raiden, Sam Gideon, Preacher(!), Commander Shepard, and a whole lot of other videogame heroes (not even counting the FF characters). So in effect, a Play Arts Kai collector potentially can make a badass army of nothing but videogame protagonists...
And as I mentioned ages back: The only way PAK can add insult to injury of me not buying any play arts figures is to make a Play Arts Kai Captain Price from MW3...