Hi belle - welcome to WebmasterWorld.
Not sure if you're aware, but the world of PFI has been through some major changes recently with PFI services from Inktomi, Alltheweb and AltaVista being closed to new submissions by owners Yahoo, with the new Overture Site Match being brought in as a replacement.
Site Match isn't PFI in the usual sense, as you have to pay an annual fee per URL - as with the older offerings - and also a click-through fee. URLs submitted this way will however have good exposure on a large number of sites.
The Site Match service has been the subject of much discussion here recently and more information can be found on the Overture site itself.
The only genuine PFI alternative is offered by Teoma and was covered recently in this thread [webmasterworld.com]. You're correct when you say there are many services out there, but the vast majority of other PFI services will provide minimal amounts of traffic, if any.