This seems to be a popular saying nowadays.
Some even use the Bible to justify it. "Judge not" the first part of Mat 7:1. But fail to read on "...,that you be not judged. For with the judgement you pronounce you will be judged, and the measure you use will be measured to you." Mat 7:1-2 then it goes on with the speck and the log in the eye...
This verse is a warning to not be hypocritical in your judgment of others. If you are going to point out error in someone's life, you must be sure that no error is present in yours.
However, you can exercise judgment about someone else's life. As a matter of fact, it is a command. John 7:24 "Do not judge by appearance, but judge with right judgement." To do so, we must have pure motives, Christ himself says "I can do nothing on my own. As I hear, I judge, and my judgment is just, because I seek not my own will, but the will of him who sent me". John 5:30
Our lives will be judged against God's word in the end. In John 12:47 Christ says: "The one who rejects me and does not receive my words has a judge, the word that I have spoken will judge him on the last day."
But judging is a tricky thing. We must have the right motive: to bring a person's life in line with God's will as expressed in His word. But we must also have developed a relationship with said person, so that our words will be received. We have to earn the right to speak to others about their life and loved them enough that they know our intentions are good, and we do not wish to cause them pain or heartache.
James warns us about judging others: "There is only one lawgiver and judge, he who is able to save and destroy. But who are you to judge your neighbor?" James 4:12
It is so easy for judgment to be taken the wrong way. For the one who judges to appear as wanting to be self-righteous. And, we must admit, it is hard to truly know a person's heart and motives. Only God knows us that well, and so He is the one that is in the best position to judge us.
But if you are going to judge someone, be sure your motives are pure, that you have developed a relationship with them that allows you to criticize them and that your own life is as Christ would have it to be. If you do not meet those criteria, it might be best you keep your thoughts to yourself.
