They tested quality in a blind test, where people were asked to choose a better looking photo originally taken with an iPhone and enhanced by one of the three methods: Apple Photo Enhancer, their CNN method, and a professional graphics artist using Photoshop. 42 people were shown 9 pictures each. From the paper:

"Although human enhancement turns out to be slightly preferred to the automatic APE, the images enhanced by our method are picked more often, outperforming even manual retouching."