I have used Hylafax and a commercial system. Hylafax works fine, but it's a pain to set up, and (the last time I looked) the Windows clients were a little obtuse. This may have changed. Commercial systems are easier to set up, but cost a godawful amount of money. As far as maintenance goes, I have seen little upkeep on either system.

Incoming faxes generally work like this: you can have specific incoming numbers routed to specific inboxes. There will generally be a generic number where all users who do not have a dedicated number will get their faxes sent to. This may be your only number. From there, it may just be a big come-and-get-it pool, or you might have someone whose job it is to forward the generic-number faxes to the specific recipients. There's not really any way to automate this. (I suppose you might be able to set up some sort of OCR, but I can't imagine it would have a good hit rate.)