I checked the Halmeter Kit...it's pretty much the same as using a voltmeter
on the existing O2 sensor, in other words, not reliable beyond 14.7:1.

The wide band units use newer technology O2 sensors that very accurately
measure air fuel ratios from 10:1 to 20:1 and use much more sophisticated
electronic control to both heat and calibrate the sensor.  They can be used
to tune a motor under high load to 12.5:1 or so, which is generally the
best A/F ratio for power.  The danger using a DVOM or the Halmeter type
kits with a standard O2 sensor is that all you really know is that the
mixture is above or below 14.7:1 ... you don't know how far above or below.

The wide band units are used for example on the Honda S2000, Cadillac CTS
and several of the new VW's to allow much more granular control of A/F
ratios and thus lower emissions / greater power.

The PLX wide band device provides emulation of the standard Bosch type
narrow band O2 sensor output so you could install the wide band O2 sensor
in a digifant system and still keep the ECU happy ... while knowing exactly
what A/F ratio you're running.  If I recall correctly, Boston Bob had
commented on the importance of A/F ratios after dyno testing his big valve
boxer in a previous post.  From what I can gather, this information is
critical, particulary when operating outside of closed loop, under high
load conditions.