I used it for a time but when I widened my stance and grip last year I found it placed too much torque on the underhand so I switched to a traditional mixed grip.
Now I take all warm ups overhand with no hook and depending on the load and how my shoulder feels I might hook grip the working sets or use straps. Ii have not had any issue when it comes time to mix the grip for a real max or in a contest.
My personal rant. on grip and straps.
If you have a weak grip you know it already and you should be working on it..taking heavy pulls with a mixed grip is not fixing grip problems it's potentially creating shoulder or elbow problems.
Work on your grip with rows, OH deadlifts in the warm up and down-sets, farmers walk etc...soon you will have no grip problems. There is no reason not to use straps to preserve your shoulders and biceps on heavy repetition pulls.