I currently live in Asia. I purchases my Magellan on a trip home to Chicago. Back home, the default map displays almost all mapped roads and landmarks, and even some I didn't know existed.
I purchased MapSend so I could have decent maps of the area I live and travel in.
Silly me to think that it would perform with accuracy as it did back home...
The mapsend features in Asia where I've used it (Japan, Korea, Thailand, Philippines, Bangladesh) show at most 10 percent of the roads and landmarks, and all of those are displayed in horribly inaccurate positions (as much as 3 KM in some cases).
Were talking big landmarks including things like International Airports here.
A very poor product but one star given because it's better than nothing.

After installing this software successfully, when I try to run the application it displays a popup stating it cannot find the CD (the CD is required to run the application) even though the CD is in the drive! So far, I have been totally unable to run this software: what a piece of junk!

I have a Magellan Meridian Platinum receiver. I purchased the Worldwide Basemap so that I could use my GPS on an upcoming trip to South America. In the 'States, I use the GPS in my vehicle as I travel, and as often as posible in the backcountry on backpacking trips.
I uploaded a region of the Basemap that included the US west of the Mississippi and northern Mexico. It only took 8Mb of memory (saved to a 32Mb card). I think this is a pretty small file for such a large area. The Basemap includes bigger cities and many major roads. It also includes topography in 200' increments.
The nice part about the basemap is that it seems to be better "georeferenced" than the default map. On a recent drive, for instance, we found ourselves driving around on water - at least according to the built-in map. Changing to the basemap we found ourselves still safe on the lake shore! I think it will be nice to have topography shown on cross-country drives. But that brings me to the down side of the Basemap.
The basemap has only a fraction of the cities and roads on it. My home city, with three highways intersecting in it, is not shown. And only one of the highways is shown, starting outside of town (though there is no evidence that the highway continues or that there is a town at all on the basemap). The default map shows all the highways, tiny cities, and even major streets. I thought the basemap would have all that is on the default map and more. That assumption was not correct. It has a lot on it, and more than the default, but not the cities, roads, and other "important" things that are nice to know while driving.
Of course, a detail map can also be uploaded (from MapSend Streets or Topo). When zoomed into a detail area, that map takes over. I suspect this is why the basemap is sorely lacking. The bad part of this is that detail maps take up more memory. Even worse is that there are no detail maps for many areas - like Iquitos, Peru. I will visit Iquitos in July, but the city does not even show up on the Basemap (Iquitos and the Amazon are both shown on Garmin's default map, not Magellan's). I bought this software for the sole reason of uploading an equivalent of the default map for other areas of the world. There isn't much topography at the Amazon, so the loaded basemap looks exactly like a blank screen! I will still get my use out of the Basemap in the US, but it is worthless for the purpose I bought it for.
I believe that most readers will be better served with the built-in default map for travelling in the US. Then consider getting the Mapsend Topo and/or MapSend Streets software first. Only get the Basemap when you have the money to burn. Even if travelling abroad, you might check the availability of a detail map before dropping the cash for the Basemap.