I feel that it is unfair to cap the number of categories on a paying account at 15. When there is no search function with the service, categories are the only way to sift through the emails. I was unpleasantly surprised to find that there is no search function, then was happy to at least have categories when I found that feature. But now I am just dismayed when I discovered the cap at 15. $60 per year is a substantial amount to spend and doesn't seem to warrant that cap.
If I find that Throttle works for me over a longer term I will obviously have to upgrade to Pro, but I feel that other features need to be addressed before I do that, such as the implementation of a robust search feature and the ability to put email streams into more than one category.