OKay...
I will answer this question, even though my subconscience is telling me
not to!
No I have not used them... but IMO I can see that they would be great benefit for a strengthening tool... you all know about the Kegel excercises right? If you don't, it is contracting and releasing the muscles that you contract to stop yourself peeing.
I was told constantly by midwifes, doctors, etc after having my baby in January to
do these excercises to tighten the muscles back up. Child birth, excess sex
and also old age can make these muscles relax and this excercise helps tighten them back up.
My guess is that you
insert the balls and with them probably being a little weighty they will try to, erm, escape?
So by contracting and releasing the muscles you will actually be letting them drop and then pushing them back... it's probably a better way than just doing the excercise.... and I'm guessing more fun too...
(Urgh, I'm soooooo gonna regret hitting submit!)