They work, but patience is a necessity. I've been reusing the DTG jumbo grips they had for a while, and since there's no substitute other than Jumbomax, I've been salvaging. I always used tape and solvent, so that's what I'm fighting. I try to get about a half inch in, dribble in solvent, ush a little more, dribble in more solvent, then rotate it around a couple of times. Repeat until your down at the butt. It should loosen up and come off.
That all sounds so dirty -- good thing there aren't too many censors here.
Depending on how the tape adhered, sometimes there's none in the grip, but a bottle brush can help get the remains out.