I used them for a few years, even to wash nappies. I came to the conclusion that they are fine for dark-coloured clothes. The additional benefit is that the clothes don't 'hold' any detergent, with the result that the cloth feels much softer, which is nice.
White stuff, on the other hand, just got greyer and greyer. Eventually I started using Ecover Laundry bleach and Hot water washes and that improved things.
I now use Surcare washing powder and have no skin issues with it. Itchy skin was what drove me to try eco-balls. I think it's the perfumes in most washing powders that give me problems rather than the detergent.