A lot of threads on here, use the search function. I think you are going to find all kinds of opinions...from ok...to lousy. I tried two different ones on my Ultra, both got lousy reception. So I went to the HD shorties on the back, and they work just as well as the whips.
I bought one years ago at Pep Boys to try it. It was a amplified one.Did everything I expected or wanted. Still working fine today, and less than $20 to boot.