The positional data from a GPSR is great, but I find I don't think in terms of Latitude and Longitude. For the data to mean anything to me, I need to visualize the position in relation to surroundings. This is where maps come in to play.
For hiking, my personal preference is still for full sized USGS TOPO maps, commonly referred to as Quads. I find the detail, and size are great aids in route planning, and for visualizing your surroundings. Quads are generally of a scale of 1:24,000.
For use in a GPS receiver, the Garmin TOPO maps are of the scale of 1:100,000. Because of the relatively low resolution of the maps, the small screen size, and the slowness with which you can zoom and pan the maps, I find a mapping GPS to be of limited use for route planning and visualization of your surroundings. The TOPO maps do however make it easier and quicker to locate your position on a real map, and once you've visualized the area with a full sized map you can still generally mentally tie the screen map into the big picture in your head. As a result, I prefer to have the GPS TOPO maps, although I'm not sure I'm convinced they're worth the extra money. In many ways, I still prefer the basic eTrex over the mapping versions. If preloaded with a route, or track, you can tell at a glance without breaking stride which way to go, how much farther your destination is, and a host of other information. The basic eTrex data screens are uncluttered, and the unit is very easy to use one handed. The click stick on the mapping units is far harder to use while on the move. It does increase the capability of the receiver a great deal however, so all in all, I'd have to say it's a worthwhile trade off.
For road use with a GPS, a mapping unit is the only way to go in my opinion. However, route planning on a GPS receiver still leaves a lot to be desired, even with the newer units that have auto routing capability. For my area, I've found that Metroguide maps, or Magellans TOPO are so inaccurate that you can't reliably use them for road navigation with GPS receiver. It's hard to figure out which road it wants you to turn on sometimes, and it will lock to the wrong road enough to be annoying if you use the lock to road option on the Garmin. I find the inaccuracy to be enough of a pain that I vary rarely use my receiver to navigate on the roads with.
Finally, be it Mapsend, Mapsource, or Expert GPS, watch out for accuracy problems with the POI data. These seem to be taken from the same data base which is prone to error for geographic features. For instance, Kings peak, the Highest point in Utah is shown several hundred feet down the slope of the mountain. Dangling rope Marina is shown way off in the dessert miles from Lake Powell. You'll find errors ranging from just a little to miles on much of the data so verify accuracy with another source if it's a spring, pond, or other important feature for you.
In my map pages you can get some of my views on the different maps, and an assortment of examples from road, TOPO, and aerial photo maps.