Thanks for the visual overview here! Loving what Sketch is bringing to the table for modern digital designers.
Regarding the quick mask, I use masking a lot, and can see this simply saving a couple clicks. Typically when I draw out a mask I won't make it pixel perfect either (yet), so I'm bullish on this feature providing a shortcut to get me to where I always am to begin with. From there I can nip and tuck to the exact dimensions.
That being said, if you need something exact, or Not a rectangle, you can draw out your shape (say a 300x300 circle), shift select both layers, and click the mask button. Bada bing.
Super helpful overview!