Johnny O,
Distilled water is often recommended for baby formula, so I'm not sure if what you heard is 100% accurate. I do understand that some health advocates say that by drinking distilled water exclusively you may be depriving your body of minerals it needs, minerals found in tap and spring water, but removed by the distillation process.
Either way, drinking distilled water won't harm you, it just may not give you as many minerals as drinking regular water. So, I guess instead of saying "not good for you", a more accurate judgement might be: "not harmful, but arguably not as good for you as regular water."
TJR