A question about workouts..

I don't know if this is in the right place, but I have a question. I have heard about a device that you strap into, tether it to the side of the pool, and it basically allows you to do the complete swimming motion without actually moving through the water. My question to you is this.. Is there any difference in the quality of the workout that you are going to get using this device and just working out the traditional way? This considering everything is constant (the stroke you are using and the effort). Any scientific facts arguing either way would be appriciated as well.. Thanks.