I have done it in the past and will most likely feel the need to do it again in the future, as my youngest DD is only 14.
Let's see, with either her or one of my other children, I have also installed a recording device on my phones without their knowledge, had another, older sibling's boyfriend hack into DD14's MySpace account, made them call me from a landline to ensure that they were where they were supposed to be at a certain time, and followed them in an unfamiliar car...that's all that comes to mind at the moment, but I have no doubt forgotten many other things.
My house is not a democracy. I, along with DH, am in charge and make decisions we believe are in their best interest. Our primary job is to keep them safe and I will go to whatever lengths I believe are necessary in order to do that to the best of my ability. I will say though, that the level of intrusion is based on my instincts about how secretive they are being and what their possible motives may be. I rarely snooped at all with my son, who is now 23. There was little need to do. On the other hand, all 3 of my daughters have at one time (or more) given me reason to snoop and I have never lost a moment’s sleep over doing so.
Sure they were angry at the time, but I am very close to all 3 of my adult children now and we laugh often about some of these things. I will also add that all 3 of them believe that I am too easy on the youngest, who is the only one left at home now. I told them all when they were younger that although I very much wanted to be their friend, my job was to be their mother and when pushed to the wall, they would find a mother every single time.