I got the smallest mattress I could find for the trundle bed, and it still didn't fit under this bed. Arg! I had to put the top portion of the bed on risers. It seems a bit unstable, but it hasn't fallen yet.
Also - I have a rug in the room it is in and the wheels do not work well on rug. It requires a bit of force to push the trundle back under, especially without risers. 
It looks great and was really easy to assemble.