Recently I decided to try again -- especially for fax receipt.
In theory one could use an email/fax service to send faxes as well, but scanning documents then faxing them is too awkward a workflow -- particularly in the absence of a sheet feed scanner. It's technically feasible, but there are problems with both software and hardware.
The hardware problem is the tougher one. Traditionally home scanners were designed for image acquisition, not document management. They haven't had reliable sheet feeders or the software to assemble pages into a document. This may be changing, but it's a slow process that's moved forwards and backwards over the past decade. For most of the past ten years reliable document scanners have started at $1,000 a unit and moved quickly into the five figure range.
Even if the hardware worked, the software I've seen isn't good enough. This task requires software that automates assembling the scanned images into a document, translating that to CCITT encoded TIFF, and uploading the result to the send-scan service (ideally through an API). I don't think there's a market to support this kind of software. Someday, perhaps in the VOIP world of the future, some vendor (Google?) will decide to take over faxing and provide the software/API solution that manufacturers will create hardware for.
So for sending faxes we will buy a dedicated fax machine, or maybe an integrated hydra multi-funtion device (but those have a reputation for buggy device drivers, unreliable hardware, and lousy Mac support). Any online service support for fax sending is just gravy (it's about 10 cents/fax with MaxEmail).
So how about receiving faxes? In theory a standalone fax machine can monitor incoming phone calls line and automatically distinguish incoming faxes from voice calls. In practice none of the people I know have been happy with this; most either end up connecting the fax machine erratically or they pay for a dedicated line. Since I'll eventually buy a fax machine I'll get to test this out myself.
So it's for incoming faxes where a fax to email service ought to work well. Nowadays the best ones support both incoming fax and incoming voice messages. Fax is translated to PDF or TIFF, voicemail to WAV (or something else). Users receive an email with an attached WAV or PDF file, or an email with a link to such. Inbound WAVs or PDFs are retained on the host service for at least 30 days, outbound faxes are not stored.
It's tough to find reviews of such services, Tidbits had the best I found from April 2005 (link to futher discussion on the right side of this page.) They liked MaxEmail's low cost ($15/year, Chicago phone number) solution. Innoport also got good reviews, but they don't offer anything comparable to MaxEmail's low cost Lite solution.
I decided both MaxEmail and Innoport looked reasonable. The latter is more corporate looking, but MaxEmail offered a free limited trial solution and Innoport didn't have any Minnesota local fax numbers. I ended up signing up for MaxEmail's Plus solution with a local fax number -- the Lite solution is a much better deal but our incoming faxes are frequently from small local educational institutions who might balk at sending some information to a remote number.
I'll try MaxEmail for a while and experiment with their voicemail service. I'll compare that to the dedicated fax machine. More later ...
Update 10/5/09: Somehow MaxEmail crept up to a $9/month flat fee. I didn't pay much attention, but that starts to add up for a service we now rarely use. I canceled it.