We use the TR/CIA dual probe resistivity kit, the result of an initiative by the Council for Independent Archaeology and TR Systems Ltd. This is a nicely engineered and very robust piece of equipment which to date we've used without any problems for several years. We've walked it the equivalent of the distance from Cambridge to Liverpool in that time. Heartily recommended.
The main problem with any kit using this type of configuration
is the drag on the cable when it catches on vegetation, which is why we usually
have an operator and a cable manager when we're out in the field. This has the
additional advantage that you've got somebody to talk to!
More technical detail on resistivity here
Resistivity surveys are very slow compared to magnetometry, which is why most archaeological reports using geophysical methods tend to concentrate on magnetometry. You get about 400 sq m by resistivity and about three times that in 30 minutes by magnetometry. Resistivity depends on the quality of the surface contact with the probes which can be problematic. The trouble is that they can show different things under the ground. These two pictures show the results of a magnetometry and resistivity survey of the same area:
The long thin strip in both cases is by magnetometry;
the lower block in the left image was by resistivity, and on the right by magnetometry.