I have one, I love it. It works pretty well in a group, the only time it alerts you to a cyclist is when they gain on you very quickly, so in normal group riding it is fine. I have mine because I have a slight mobility issue meaning I find it difficult to get a full look over my right shoulder. I still need to perform a quick shoulder check before a manoeuvre but I don't have to do standard checks whilst riding. I don't use it on my commute as it would go off a lot and to be honest there are always cars around me when I am not on the cycle path.
One bonus - because it is ANT+ you can have your friends connect to simultaneously so the whole group gets the alerts.