Below are some items we purchased specifically for our trip to Mexico, but found they were not so useful (and in at least one case, useless).
Honda Generator EU2000i:
We only used the generator twice in our two seasons in Mexico (and never on the San Francisco-San Diego legs). Our solar panels gave us the charging we needed.
The first time we used the Honda generator was in Turtle Bay, on the way down the Pacific Coast of Baja when we had sailed most of the way from Ensenada (3 days/2 nights): it was cloudy, and we used the autopilot too much (rather than Hydrovane). Then it rained all day in Turtle Bay. The Honda Generator helped bring the batteries up without us having to run the engine. (As it turned out, we stayed 4 nights in Turtle Bay, and the sun came out for the final 2.5 days, allowing the solar panels to fully charge our batteries.)
The second time we used the Honda generator was much later in Puerto Escondido. We decided to have a little luxury and have hot showers onboard (the marina was a long dinghy ride away; and besides, its showers were not working). So actually no real need to use the generator.
Perhaps if we had had a watermaker and had anchored out more often, we might have needed to use the generator. Nevertheless, even after anchored out for several days and before moving to a new anchorage, we didn't need the generator. (Note: another reason the generator wasn't needed is that one has to motor a lot in Mexico -- so lots of charging and hot water.)
Yachtsman Mexico to Panama Chart Book (7th Ed):
Note: these charts are not "official" charts, thus they have the same legal status (though not utility) as C-MAP or Navionics.
At US$90, this chart book was, for us, a waste of money. We do not recommend you buy it.
Blue Performance Wind Catcher (wind scoop):
We like wind scoops, just not this one. It has a bug screen on it, which effectively cuts air circulation to perhaps one-third (I suppose one could cut out the bug screen -- would be nice if screen could be removable). On the positive side, the Blue Performance Wind Catcher is multidirectional (i.e., does not need to be moved when the wind direction changes while at a dock).
We had better airflow using simpler wind scoops such as the Davis wind scoop (or even a tarp placed at an angle).
Wind scoops are problematic, however, when there are bugs outside (not uncommon in the Sea of Cortez) -- hence we purchased the B-P Wind Catcher with its screens. Too bad this significantly cut down the air circulation.
Collapsible/Folding Hand Cart:
We only used our "Magna Cart" once in two years (to get propane in Ensenada). Not very strong and problematic on the rough sidewalks/roads of Mexico.
Charlie's Charts & Rain's Mexico Boating Guide cruising guides to Mexico:
Inclusion of these cruising guides on this "less useful" list is a little problematic, as we definitely used (and needed) these two guides for the Pacific coast of Baja (there were no other reasonably up-to-date guides available that cover this coast). These guides provide waypoints and anchorage coordinates, as well as chartlets. Each to their own, but in our opinion their chartlets did not give us confidence when navigating near the coast.
Once we rounded Cabo San Lucas, and started using the Blue Latitude Press cruising guides by Shawn and Heather, we rarely found Charlie's Charts or Rain's Mexico Boating Guide helpful. Eventually, we stopped using them. So, we found these latter guides "less useful" specifically for the Sea of Cortez and the mainland coast of Mexico.
Baja California Cruising Map
Available online and from some marine stores in California, at first glance we thought this map and its GPS waypoints would be useful for us. It wasn't/they weren't. Although it provides GPS waypoints for anchorages, the scale of the map is much too small (i.e., no fine detail) to indicate where in an anchorage the waypoint is located, even in an approximate way (no features are visible). Given that many areas are already poorly charted, having a waypoint with no additional information does not inspire confidence. (The lat/long coordinates for each waypoint are listen in large table on the map. It would have been nice for the authors to provide a way to purchase and download an electronic version of these waypoints.)
We only used this map to give an idea of where a location was in the overall picture of the Baja California peninsula. And only occasionally. Nice as a picture on the wall but not so helpful navigating.
The above is not a comment on the accuracy (or inaccuracy) of the waypoints -- we just didn't use them. The map does provide passage waypoints which might be helpful for those considering the Baja Bash (i.e., going north).