pucho812,
I don't "use" them per se because I am just getting into understanding circuits, but I do have a JTM-45 clone that has mustard caps in it. The amp blows most anything clean out of the water. I can't say it is purely due to the caps, because the guy who built it did a beautiful job inside and out. I am sure its a combination of factors including choice of transformers, tweaking the design, and the skill of the builder. I think these caps are to use in vintage clones or re-furbs.