I use efax. Cost is $19.95 a month. Every once and awhile I will get some over usage charges - but very rarely. Note that the $19.95 is US. So if you like $Cad right now relative to $US, it is actually cheaper than myfax.
My real estate partner uses myfax.
No issues with either. Great services. Real liberation knowing you don't have to hang out by fax machine - or be dependant on your fax machine receiving another fax - or - be turned off. You can pull faxes off your e-mail - at cottage, on road - anywhere. Its also close to same cost as a dedicated phone line - without the flexibility.
Once you sign up, your life will never be the same.