I used the field paste - it is really really good and smells really nice too. I used it more when the feet were good to prevent the thrush and used iodine when the thrush flared up. Best if you can apply it to a washed and dried foot or when the farrier has just trimmed.
I used the paste when they were trialling it and actually found it very effective, it isn't something we have had to deal with since but IIRC it cleared it after just a few applications.
Field paste is great. I used to use hydrogen peroxide but was told that it isn't good as it kills off some of the healthy parts of the hoof, I think! Can't remember exactly sorry, but I stopped using it and went to field paste. Great stuff.