When built this shelving unit is very sturdy. 
However, the plastic parts that form the protective feet are not really man enough for their intended purpose. I replaced them with custom made hard wood blocks that are much more suited to the task, this makes a much more stable base. The same parts are used as safety top caps for hiding the sharp edges and corners of the vertical bars, here they are more successful, but are too easily dislodged. 
Building this shelving unit as the designer intended, with each male metal tab passing through each female loop entering and exiting parallel to the vertical bar, is far from a trivial task for the novice constructor. However, patience and perseverance are ultimately rewarded with a very sturdy and good quality construction.