I've had Garmins, Magellans, Smartphone apps, GM (and Harley) built-in "Navinfotainment systems.
By far, I prefer Garmin. With Garmin, not only can I load many web-available custom POI files, but I can write my own using Garmin-provided software.
If you can set up and fill-in a spreadsheet you can make your own POIs, complete with proximity notifications, custom icons, and custom sounds. You can even write your own "tours". Of all the systems I've owned, only Garmin allows this.