Really great app. So far I've sent a handful of faxes through my Sipgate.de account (which doesn't even have the "Plus" fax option), and it's worked beautifully.
When I had what I thought was an issue, he returned my e-mail within a day and encouraged me to continue providing feedback to improve Fossi. He's been extremely courteous, even in the face of poor spellers calling his free app "a waist of time."
Since Apple's taken the modem out of Macs, this is a fantastic solution to the problem of convenient faxing from the print dialogue. And even if you've got an older Mac with a modem, this obviates the need to stay plugged into the phone line.