I have not rented a rock grapple as yet, waiting for two farmers to let me try theirs, but they are still trying to finish planting and have not delivered. One friend has a Virnig for sale, http://virnigmfg.com/products/grapples/
Virnig is a purdy good one..... I've had one for ten years or so and its always ready to work. I got mine from Zieglers.