I think the Water Magnet is great. I have a polished black truck (GMC fairly soft paint) and I haven't noticed any scratches induced by it. If you are scratching the finish perhaps:
1- you didn't clean the vehicle properly and there is still debris
2- perhaps you scratched the vehicle during the wash (most likely)
3- debris was introduced in between washing and drying eg. windy day
4- you aren't storing or cleaning your Water Magnet properly
5- you are using force while using the Water Magnet. You're not scrubbing the water up. You shouldn't be using any force at all. If you have ultra soft paint, then use a blotting technique.

I'm sure there are more possible reasons, but look beyond the product.