I have never actually worked out with one, but I had the opportunity to try one out.
Some people report good results with it, but personally I didn't like it. The reason I didn't like it is that the resistance it offers is uneven, easier at the beginning of the rep, harder towards the end. The difference in resistance is very noticeable. The reps are either way too easy at the beginning, and ok towards the end, or ok towards the beginning but so hard towards the end that it is nearly impossible to finish the rep.