FAX STF PRO X does everything it says it will do, and certainly provides a good interface for faxing OS X. It has a number of features that make it an improvement over the built-in faxing in X, like archived sent faxes and notifications.
HOWEVER, my beef with this company and its software is its crazy policy of not allowing downloads after 30 days of a purchased product. You heard right. You BUY the product, and you change computers and don't have a backup, and even though you have a valid purchased serial number, they will not let you download the product again.
So here i am with TWO serial numbers and no product (i know, shame on me for not backing up, but...). Ergo, i'm not buying their product again, no matter how good it is.