Sketchbook Pro for mac is AWESOME.
It is not and cannot replace Photoshop.
Sketchbook Pro is for sketching and painting.
Photoshop is for a million things and sketching and painting are like a few rooms in a mansion.
It’s not Apples to Oranges, it’s Apple’s to Fruit Stand.
Sketchbook pro is lightweight, cheap, tablet centric, (every command is pen-input friendly, which you can’t say about photoshop) and has some very cool stencil tools that work much like you would use a stencil in the real world, also very unlike photoshop. IF you have it on a mac you can use iCloud to sync it up with the iPad version. I’ve taken drawings done on the train and one-click opened them on the desktop for uprezzing and finishing. Pretty nice workflow. Not sure if the price came up but I got it on the Mac App Store for $40, and the iPad app is only $10. Also, not sure if this is true for PC’s but there is a Free Sketchbook Pro Express app available to start playing with.
If you want to collage a bunch of stuff in with your painting, or warp layers, or work with hundreds of layers grouped into folders, or if you want to take your color image into LAB or CMYK color space, or check a histogram, or batch process 8000 images, or have amazing filters and plugins, then go with Photoshop.