I do them, my grip strength has gone up considerably since I added them in (I call the ones I do forearm curls). But I also do them a bit different to normal, I grab my barbell and stand up letting my arms hang down as if I was gonna do a curl but than I lower the bar down my fingers and bring it back up by squeezing the bar hard and curl it as far as my wrist will allow holding for 2 seconds and repeating. I do 50kg on the Texas bar, and my hands are on the knurl where it starts. I do a couple sets of 12-15 reps. I got showed this off a guy who once was a powerlifter in the old days and he said that's exactly how he managed to bring his grip strength up besides doing heavy deads etc. Its a great assistance exercise and I can attest to them working because my grip strength was very week before, and held me back considerably on my Deadlift, but now its way better and my last 1rm I had great grip, now I just gotta work on other areas.