I looked at this with my co-worker--he ended up purchasing one. The major drawback I had with it is there's no way to call back/intercom TO the door bell.

So if someone presses the doorbell and you don't have your phone within arms reach, but maybe in the same room, its possible for the alert to time out and there's no way to talk to the person at the door. Similarly, if you hear something outside and wanted to use the doorbell's camera to get a quick view, it's not possible to initiate a connection from your phone.