they work great...
on smaller boats, you can get the same effect by using a figure eight rappelling ring... this is what i use and it works on the same principle and without the cost... by tighting the line, you can "lock" the boom in place, or adjust the line so the device will act as a brake and let the boom swing over easy and slow, rather than slam across.... it also works upwind in the same manner. no more worries about how fast and hard the boom is going to come across...
or you can slack the line completely and the boom will function like the device is not there at all... its nice to have options.