FaxMail is simply an interface to the Internet's Email-Fax gateway. When you send your email, a central server figures out whether the desired number is covered by the system. If so, it forwards your email to a computer that has agreed to provide a service to that number and that computer places the phonecall.
As such the service is as efficient as email is. Several attempts are made at delivering faxes and in all events the outcome will ultimately reach you via email (so you don't have to guess if the fax got there or not).
As with almost everything on the internet, it is up to each individual to make sure that they use the service responsibly (So no faxing of PhD theses and the like! ). Several ps files are better sent in succession rather than in one big email. This is likely to be quicker, the only possible drawback being a replication of the cover sheet at the remote end. It is perhaps wise not to send material of a highly confidential or important nature via this means. It's up to you though.
The success of the Email-Fax gateway depends on individuals and organisations agreeing to provide the local fax service. If you have a fax-modem and would like to help, check out http://www.tpc.int/servers/jointpchowto.html for details. www.tpc.int is also the starting point for information on the Email-Fax gateway.