A local 14.1 league plans to use them. I think it will be in every match in which a player requests it but I'm not sure. There are some players who take two or three times as long to finish a match as others. Related to which, I've seen a player take eight minutes to shoot one shot at straight pool.... Are you aware of any leagues or tournaments that use a chess clock? ...
How should breaks be handled? I think that if you take a break when it's your turn, there's no problem -- your clock will be running. Maybe it should be allowed to start a break on the other player's turn, but if he ends it before you get back, your clock will be running. Usually it's a no-no to take a break on the other player's turn.