I used a plastic spinner and it helped me tremendously. I used to call myself spin challenged as I couldn't even do a basic two-foot spin. It took me over a year to get three shaky revolutions on a one-foot spin, but I couldn't cross my feet toe save my life. Then my coach suggested a spinner. I couldn't believe it, but that plastic thing revolutionized my spins. I noticed real improvement in about a week. Withing a couple of months I had a decent scratch spin - about 15 revolutions. But I can only do about three revolutions on the spinner. Still it helped.
I agree with what was previously said about the importance of technique, proper blade position, etc. You surely need that.My problem, however, was coordination or complete lack thereof. I also used the spinner to practice my back spin. Unfortunately, it's still on the inside edge though I can get several revolutions (probably, seven or eight). So of course, it's not a substitute for on-ice practice. But at the beginning, I couldn't even do one revolution on a back spin. As for the other spins, I can't say the spinner helped a lot. I can't do anything that would look like a sit spin on it. As for the camel spin, I can do one revolution, then I step out of it.
Overall I think a spinner is good for developing balance and coordination.