Made my own for $25
Got my money's worth and some. Seems ok for some exersises, worth having as a tool to "mix it up" with core work.
I do think there are some things that you can do on it that may overtax the smaller stabilizers if you are not ready for it. Seems like you need to have a good "feel" for what is the right position.

Just grabbing and going to town on it may be bad for some people.