I've never actually heard of this service. Two things though:
I'd be really suspicious of this 'free' service. They say it is 'ad-supported', which sets off alarms for me. I hope that doesn't mean that they send tonnes of fax-spam to the recipients of your faxes (or still spam them when they send you a virtual fax from their machine).
Second, they say they off phone numbers in 65 countries. It may be important for you to have a virtual number in your local area code, if you do a lot of local business. For instance, if you live in Rochester, NY, but they only have numbers for NYC, if anyone in your hometown (Rochester, in this case) send you a fax, they will have to send it long distance.