I think they are very effective.
I use them for Bent Over Barbell Rows. Cannot execute the movement properly, correct form, on heaviest lifts without them.
Never use them for Deadlifts. Use Chalk instead. Never had an issue with grip on heavy deadlifts or rack pulls as reps never go over 5 . . . .or 3 for very very heavy.
Use them sometimes on heaviest sets of Weighted Wide Grip Chins . . . .again so I can concentrate on the form of movement as opposed to holding on as hard as I can. I will still grip the bar as hard as I can to activate muscles.