The unit plugs into the Diagnostic connector and has a Buzzer/Light for signalling the codes. It has a selector for choosing Engine (incl Trans on I6), Trans (V8 and some EB I6's), ABS and Airbag. Also has a Power and Fuel Pump indicator. It has a switch for initiating the test sequence.

Yeah, one could do the same with a multimeter.... main advantage of this unit is that you can just plug it in and Read and Clear codes easily.....i.e. convenience.
I am also assembling a user guide incl code tables. Again, much of that info is already available on this site... I'm just packaging the info up in one convenient A5 sized booklet..... but using the Ford Workshop Manuals for sourcing the info.

If one is used to using a meter or lamp, etc then fine..... I originally made one of these for myself and use it fairly frequently.... less fiddly.

I had looked at making a unit to decode and present the codes on an LED or LCD display. Another option would be to send the codes to a Laptop or PDA using Hyperterminal. May do one just for the exercise. But I could not see people spending $300-ish on such a unit. Personnally, the simple little unit I made so far has served me well, and I trust, the guys who have bought them so far (have not heard any complaints).

Anyway, enough rambling.... hope that clarifies.