If you'd like to see it in action, check out the video below.
Instashare is available for Mac for $2.99. It's free for iOS devices (but you can upgrade to the no-ad version for $0.99) You will need to purchase the Mac app to allow transfers to your Mac. It's also available for Android and will be coming soon to Windows.
I will still be using Dropbox to share folders of images with family and friends but Instashare will now be my go-to app for sharing between devices.