I have not used the Ultra Nok in particular but I have taken quite a few off of customer's bows. I have also personally used a similar device called the 5 star nock.

Most of the negatives that I have seen are an increased noise level on some bows....string/cable wear from contact upon release and a decrease in speed because of the increased weight in the middle of the string. An occasional broken monofilament serving or two as well...but then monofilament servings are less desirable to begin with.

I am glad to hear that yours is working well for you but do not think I would personally use one for the reasons I mentioned above.