I like that it allows me to easily track all of my incoming calls, as well as outgoing ones (including those made by other companies).
It's easy for customers with multiple numbers in one area/office or even home office situations because you can assign each number their own "fax". The only downside would be if there was no way around having them sign up so they could actually use this service without being forced into paying first! If your business has many phone lines then definitely consider getting more than 1 account since it will help keep things organized better which makes life easier when trying to manage large volumes of work from different sources at once time.