Certainly you can get free better topo maps here than the built in maps. However, the T models also come with 100K DEM (Digital Elevation Model) data which some features require. You can only get DEM data from a T model or Garmin 100K topo or Garmin's 24K topo (which have 24K DEM). Us map-makers have not been able to figure out how to make DEM data to load on your GPS, so you cannot get DEM data from gpsfiledepot.com.
While one may still decide the DEM data is not worth the extra money, it needs to be weighed in the decision. I thought it was worth the extra money when I bought my 550T.
DEM:
To show shaded relief, the GPS requires DEM data. If you have DEM data on your GPS, it will show shaded relief on any map (including the ones from gpsfiledepot.com). However, shaded relief makes the GPS significantly harder to read. However, sometimes it can be nice when looking at a map to see if the trail goes uphill or downhill. You can turn it on if you need it. Without it, usually you can tell which way is up, but sometimes you have to click on topo lines to figure it out.
The 3d viewer requires DEM data
With DEM data, the GPS will add elevation data to a track you load and follow on track manager. This allows you to view the elevation profile of your trip. However, I use
http://www.gpsvisualizer.com/elevation to obtain the elevation profile as the DEM data there is more accurate than the built in 100K. But it is nice to have it just in case.
These are the two things I like (and use) the most:
With DEM data, the GPS will add elevation data to a car route. So you can see the elevation profile of the route you are driving.
With DEM data, you can click anywhere on the map and one of the pieces of information about the site that will be provided is the elevation. While it is based on 100K DEM, it is reasonablely accurate.