I was surprised at how sturdy this little foot rest is. I've now been using it nightly for a little over two weeks, and it doesn't flex, warp, or rock back and forth underfoot while I'm playing. It's small, about the length and less than the width of my size 11 shoe, which I thought would be an issue, but it's not at all. In fact now I prefer it that way and would probably not want it to be any larger, since it does the job.
It's also adjustable, though as you raise and lower it the pitch of the rest changes slightly. The top rubber pad is very grippy, which is also a plus.
The only thing I want to add for anyone reading reviews and deciding whether to buy one of these or not is to absolutely buy one. You cannot thoroughly enjoy your classical guitar without the extra comfort one of these foot stools designed for your comfort (best resting foot angle, height, non-slip, etc.) gives you. A slightly uncomfortable foot translates all the way up your body to diminished enjoyment. This is nothing you comprehend if you think you've been doing fine without one of these.