I do use it since last autumn and consider it definitely a brilliant tool/idea. I use it only in case I cannot meet the guests personally because lets face it based on statistics and my practice too, the majority of the guests arrive weekdays, in the middle of the day when I am normally at work. It is really convenient to use the keybox to give access to the house.
One more thing: did it happened to anybody that the guest sent a message just before their arrival that the plane/train/bus whatever has delay? Well in this case you could go home and return later OR use the keybox.
Yes, some of the guests have difficulties in entering the PIN code others don't depending on their skills or some other factors but until now I never had to go in a hurry to open the door.