We've been struggling with Garmins. We can upload imagery and data using a phython script to create a KMZ file then drop it onto the GPS. It kind of works (much better on the newest units, barely on the older ones) although it is sluggish and rasterized. We also convert data to GPX files but need to simplify and use individual lines or single outlines. Anything more complex, multigeometry, holes, does not work correctly. Adding many individual simple tracks to the display (and setting different colour properties for each) on the unit one by one is a PAIN.
I've used Trimble a decade or so ago and remember that something like this was easily accomplished with additional expensive software (ArcPad).
Is there any GPS unit out there (i.e. the new Trimble Juno series) that enables users to easily upload geotiff and shapefiles without an additional software investment?