These are designed for wire nuts with SHOULDERS. They will not work on wire nuts that are round across the entire circumference.
That being said, it does a great job with shouldered wire nuts. Pop the nut on the wires, twist a little to get it gripped, and then give the Spin-Twist a rev with your drill. Solid connection half a second later. The only gripe is that it's a bit loose with the tolerances, but I believe it's by design to prevent over-tightening.