The resolution on the gps was as high as on the pc, so at least for not so large areas it is an option. Used it this week to put a aerial image with some polygon overlays for a few hundred hectars onto a garmin. I think it depends on the area you are exporting what the outcome resoultion will look like (1 megapixel per tile times max.

It will take quite long but will save a kmz that you copy into your CustomMaps folder (not sure about the garmin restrictions here about multiple custom maps and tile limit, so best try with only one kmz first). Start the tool from the toolbox (in the catalog where you downloaded it to) and put the 90 or something in the tilenumber. Just put your topomap into arcmap and zoom to the extent needed. Maybe that was why it did not work for you earlier. That are 1024x1024 Pixels and max 100 tiles in a kmz(some models can do more, but this is not documented but tested). Locate the custom map (named according to step 5) under Places (left of the map).

The advantage of this toolbox (beside nothing necessary to install, free and a interesting python script) is that it honors the garmin built in restrictions. kmz that is also readable by your GPS (the garmin webpage tells that your GPS should be able to). Its a toolbox with a python script that exports what you are seeing on your active data frame to a.

There is a tool where you can export your active dataframe as a garmin custom map.

Garmin custom maps arcgis