I've been using Updox for about 3 weeks. I like being able to mark up the faxes and file them directly into AC. Also you can customize categories with Updox when you file stuff into AC.
I've never used paperport but have to say the transition to Updox was pretty easy. The customer service is awesome.
As with Bill, I sometimes look at faxes in the evening when I'm out of the office. So it makes it pretty easy to work when you are away from the office.
You only get charged for incoming/outgoing faxes. Scanning stuff into updox does not count against your incoming/outgoing pages.
There is a two week free trial. That's what we did and we signed up afterwards.
