John,

Practical Sailor (magazine, or www.practical-sailor.com) tested one of
these types of controllers, don't recall if it was this model, within
the last couple of months and confirmed it's not snake oil. I
imagine the manufacturer can find the article and get you a copy.

Practical Sailor has a history of recommending and trashing products
on a fair basis, and is very reliable.

I recall the explination in practical sailor was something like Bob
posted (DC-DC controllers) above, rather then typical contollers that
throw away the extra voltage above 12.6 (in the form of heat) from the
solar panels, these types of devices will convert the energy to a
lower voltage (and higher amperage) and give your batteries more of
the watts produced by the solar batteries instead of throwing them
away as heat via a voltage regulator.

There are not many caveats, but one important one is that the
controller does not provide a gain in all conditions. Solar Panels are
only producing voltages excessively over 12.6 during sunny days
without excessive shadows (e.g. mast, etc.) over the panel.

Dan