I used them a while ago, and found they improved my strength quite a bit. I do go back to them from time to time to top up, but I've found too much work on a fingerboard just agravates problem of the long standing problems I've got with my fingers.
Not so sure about endurance though, we've got some 40 move circuits down our local wall which you can link together in to a shocking 120 move circuit that goes at about Fr7b.