I think the vehemency with which this thread is being discussed shows that my comments are needed. The biomechanics of the squat alone show that squats need to be executed well in order to make them safe for your joints and back. I'd ask people to read, among the many I have made, this thread:
I've been training for 15yrs and only learnt to squat properly in the last 5yrs. I sorely wish someone had shown me how to squat properly 15yrs ago rather than having to "work it out for myself". Now in the day of the internet the "work it out for yourself" mentality still predominates when we have much better information.