Efax has received a lot of customer complaints. There's a write up over at http/computerfaxing.org that compares some of the other services, and talks about what to look out for. MyFax got a high review, along with ringcentral - but I'd watch out for Efax.
The TrustFax service as such is okay but they will try to overcharge you. And also when you cancel the service, Level 1 customer service will promise you the refund for unused months already charged upfront, but Level 2 customer service will deny the refund. So think twice when cancelling the service.
A question I have seen come up many times is why there are not simple pay as you go online fax services. There is one at LowFax.com There is NO subscription needed, so if you only send occassional faxes and don't want to be tied into monthly fees, this is a great solution. Lowfax.com is an easy to use, web based site to send faxes virtually anywhere in the world.
Efax parent bought trustfax and MyFax, so they are now all owned by J2 Global and have similar service drawbacks. Might want to look at some alternatives like RingCentral or MetroFax. Faxcompare.com is a website that compares those and a bunch of other online fax services. Probably worth a look.