I had many of the same questions about the spinners. I got an inexpensive one on eBay to test on a short trip (3 days) from the US to the UK where I didn't need a suit. I absolutely loved moving it around in the airport. There was so much less strain on my wrist. It took a little getting used to, but it was definitely easier.
But there is also a minus side. The interior room is reduced to allow the wheels into the overhead. And the case is a bit tougher to use on only 2 wheels, as in going up steps.
Since I often use EU carriers, there is a frequent problem with the definition of proper sizes. I have seen many folks get their US-legal cases rejected by the EU airline agents.
I will use the spinner when I need less stuff; but the 20" spinner definitely holds less than the 20" roller.