Bump - This is AMAZING. Took out all of the collar ring on a white cotton shirt, striped white shirt and pink shirt as well.
I used Oxyclean recently and it worked fantastically. I sprayed on stain remover, brushed the fabric a bit and let the shirt soak overnight. From there I put the shirt in the washer and it came out completely like new... I was pretty surprised.
Can't speak to that product, but I can tell you that Shout Advanced Gel gets rid of ring around the collar brilliantly -- no oxy anything required, and therefore retaining the luster on your MOP buttons. You have to use the fabric brush at the top of the bottle to really work the stuff into the stain.
(I do use the Spray N Wash spray for non-ring-around-the-collar stains and it's great.)