Not many but very few Twitter users would like to know who are their influential Twitter followers.
As soon as we get notified when someone follows us, we can see his/her importance, relevance and influence.
However, these metrics might change over the time and we certainly won't keep track of them excepr reading their tweets occasionally.
Here is a website for finding out this valuable information. You simply log in with your Twitter account and it does the rest.
However, you get very few statistics as a free member. If you pay, you get to know the full statistics.
I am not going to pay to fruji.com