As I already had the Photography CC subscription, I just had to click install. Seems to work just as before. While it kept my export settings, it lost my display settings (ie: showing cropped res and exposure info briefly when switching images). It made duplicate upgraded catalogs as Kayaker pointed out.

For those on OS X, it says it needs 10.9 to use the GPU (I'm still on 10.8.5 for no particular reason other than "if it works, don't mess with it"). What sort of speed improvement are you seeing? What's your CPU and what's your GPU and RAM on each?