We bought two, and they are so incredibly sturdy. And well built.
A 6 inch mattress should be your max thickness when buying mattresses for the trundle. With sheets on, a 6inch mattress leaves just enough room to push the trundle underneath. We have 10 inch mattresses on the top beds.
The only change that i wish could be fixed was that the metal slats were closer together on both the twin beds and the trundles underneath. When Making the beds and putting a flat sheet on over a fitted sheet, if you tuck the sheets on tight under the end of the bed, they tend to fall out between the slats. I feel like if the slats were closer together then it would hold the flat sheet in.
A little frustrating to have it fall out all of the time. It throws off my neat and crisp look. But i love them and my kids love them even more !