I've used the Lewis lead quite a bit prehospitally. It's been very useful. For example, the other day I had a patient with a narrow irregularly irregular rhythm in the 150's. A standard set of leads was unhelpful in determining if it was sinus with frequent PACs, MAT, or AF. A Lewis lead helped amplify atrial activity, showing associated p-waves of only two morphologies.
I can't think of an instance when it's changed my treatment plan, but by using it, I have been able to be more definitive about rhythms, and I can only imagine that that will eventually lead to a change in course.