I actually liked those and never had an issue with them staying down. But that was back during the first half of GWOT. Maybe they got worn out like all the M9s. I could definitely see them wearing out, given the spring loaded design.
The issues I heard about them were people over-torquing and breaking them when attaching them. And also them snapping off because big-Army thought they could use them as primary sights and people left them up during field exercises.
