Sorry, but he photo provided is not the best example. The body needs to be more straight, his lower back is curved in too much, this can lead to back problems in the future. He should have a more neutral spine. In the above pick, his bum is sticking out just a tab bit too much. With such an exercise, in regards of the back, correct form is crucial.
The exercise itself is good, but, if you are training regularly, then all the muscles you need from your core are being imployed. IMO.
I will check into this.