Switched from eFax to faxpipe
I switched to faxpipe from eFax after getting lots of misdirected faxes that were intended for the company who previously had my number (that company was terrible about disseminating its new fax number).
Since I fax infrequently and was getting charged for all those misdirected faxes, I looked around for a less expensive online service and chose faxpipe on the basis of price, fax service, and customer service. The big win with faxpipe is that since it is email-based, it will work with any operating system - Mac OS, Windows, Linux, Solaris, etc. - no special fax client software needed.
The only downside to that approach is you have to compose the cover page and email message yourself, and I occasionally mistype the fax number. But at something like 1/4 the price I can live with that.