I recently purchased a Garmin Montana 600. I have successfully downloaded the Idaho and Nevada Topo maps. They look good but I have no way of comparing to the Garmin 24K maps to purchase. Are the Garmin maps really worth the money? I would much rather contribute to this site as I greatly appreciate the service.
Garmin maps have a couple features not available on any third party maps The first is DEM data that produces shaded 3d terrain. Garmin has never revealed the details of their map format because they consider it proprietary. All the maps on this site were created with reverse-engineered tools. And none of these tools support 3d terrain. DEM is a Digital Elevation Model of terrain, such that every point on the map includes elevation data. You would need to decide whether that adds anything for you.
The other big difference is that Garmin's 24k topo series maps include the same routable road data as their City Navigator maps. So they are capable of giving you turn-by-turn driving or walking directions. Only a small handful of the maps here are routable, and they still won't support all the features of Garmin's proprietary maps. The Garmin maps also have a rich POI database of restaurants, gas stations, stores, etc.
I also have a Montana 600. I have a copy of Garmin City Navigator on it for times when I want to use it in the car. I also have the Garmin 24k NorthEast topo although I almost never use it. I make my own maps, so that's mostly what I use. :)