I would imagine that these are like those other locks where you can create your own combination. Once you set the combination or the key, it can't be changed. Thats it. So no one else would be able to change it.
As far as security goes, I use those alarms that you install yourself. I can't remember the name of them but they are nice. The alarms are two pieces. One piece gets installed on the door frame and the other piece gets installed right next to it on the door so that they touch. If the alarm is on and you open the door it will sound and it's very loud.
You can install them on windows as well. You could even install them on cabinets where you keep chemicals so that it will sound if a small child opens the door. They are very handy.
I'm gonna look and see if I can find them on the net.....
Aha! Thats what they are. I see there are lots of names for them though. You can purchase them at hardware stores and I know I bought mine at Family Dollar. You can get two or three for something like $10.