Well after watching 52 people on this forum read but offer no reply I called Garmin support.
You should realize that GPSFileDepot is just a repository for the work of many different authors. There are no standards for "GPSFileDepot Maps", each author does whatever they like.
I try to help when possible, but have no familiarity with that map, so I didn't reply. Have never been to Hawaii so I wouldn't be able to evaluate how good that map is anyway. Generally speaking, I'm not very fond of OSM maps in my part of the country (NorthEast US) so I don't use them. The roads are there, but the names are often very different from the signage you will see. It's not necessarily "wrong", but the OSM maps will have road name, like "Main St" while the signage will show a number such as "Route 45".
And the OSM maps don't have anything resembling the set of POI's (gas stations, restaurants, shopping centers) as real Garmin maps. OSM also lacks the speed limit display found in Gamrin City Navigator, and you won't get lane assist. These features work on the Nuvi and certain high end Garmin handheld models like the Montana, but not on units like the eTrex or GPSMap series.
So it depends on what you need as to whether they are a "good alternative" to Garmin City Navigator. If I were driving around a city, I would not want to depend on OSM - but that's just me. From what I understand, the EU OpenStreetMaps are better than what we have here.
Sounds like a great vacation - enjoy!!