Contruction seems fairly solid, and won't seem like it would break in normal use, but if kicked hard enough, the frame could likely bend.
The highest 2 settings are a bit shaky, but it should keep it's balance if foot is placed well on it and you don't move your leg around much. Rubber feet keep it in place so it doesn't slide around.
All in All, it does what it's advertised as. Other than being wee bit shaky, it's an ok foot rest.
EDIT: After couple months of having it, it has started to rock alot more. I don't know has the frame bend a bit, but the legs seem to be bit off which causes it to shake more than before. It still does it's job, but personally would recommend looking into other foot rests.
EDIT2: After over a year of owning it, it has frustrated me countless times how easy it falls over and opens up and you need to set the height again. Can't recommend.