We have one of them that we use on vehicles that we don't have the full
diagnostic tool on.  It's sometimes necessary to clear codes before
programming keys in some cars.  We use VAG-COM on VW, and have a dedicated
Ford scanner (NGS tester)that I use on the locksmith Ford van and
programming Jaguar keys.  The little scanner does well, though, to read and
clear codes.  The book doesn't go very deep, so for real diagnosis, you need
a manual to understand what the codes really are.  For the generic OBDII
codes, it will somewhat direct you to a problem area, though.  Worth the
$40.
Steve Sandlin
Steve's Lock Shoppe
'84 Wolfsburg Westy
'79 Transporter
'00 New Beetle