I got them many years ago. The thing that I noticed is that they'll work with just about anything IF you lighten the weight. They're really good for getting more bang out of lighter weight exercises. Now, that doesn't mean to do high reps. The reps will be just as difficult. But in a different way. That's the point - to challenge the muscles in a new way. Trying to use the same weights you normally lift will not work on most stuff, imo.
I think the best way to incorporate Fat Gripz is to use them on one or two exercises each time. Like, if you have 4 shoulder exercises that you regularly do, use the gripz on 1 or 2 of them. And don't worry if that the weights are lower. The other exercises will not only pick up the slack, but may actually go up.