When ever I hear coaches using the wrist band letter/number calling I always want to jump up and yell "BINGO". I am still a little old school using numbers for pitches and different hand gestures or body movements for offence.
We bought them two years ago. They were a hassle. Seemed like every weekend someone didn't have hers. We have the QB style with the Velcro flap. Three card system. Each card was printed in different color ink. And each card had a different code for the play. For example, black 32 meant drag bunt to 3rd base side and red 7 was the same play except it was read off a different card. We haven't used them lately because its a little smoother to just use traditional hand and arm signals. Plus when I'd call "black 32" the other team would write that down and it then became a game of them trying to pick my calls. Hand sings are tougher to pick when we add indicators and I like to give the steal signs with no one on base for example to confuse the other team.
I have used the wrist coach for 3+ years and we also mix in traditional signs as well. I have seen people writing them down we make the cards double sided to counter it. Then we flip them over it really messes with the other team its fun when you swap a bunt for a H&R and the 3rd baseman charges in and the hitter smacks the ball with a full swing they get skittish in a hurry! i just laugh at them!!