MM uses raster versions of the printed charts, which they license from the relevant authorities.
It is a fair choice for the UK but rapidly becomes very expensive if you go to other countries, or if you want to buy other maps, e.g. the Ordnance Survey set. I went to a fancy bookshop in Oxford the other day and it was stacked up with CD maps for MM, and one could easily spend £1000s on the data.
Another option, where you supply your own scans of whatever maps you want, is Oziexplorer (
www.oziexplorer.com). I have scanned in the CAA charts; each one is about 50MB and I can get the aviation charts for most of Europe on a single 1GB compactflash cartridge. OE works out far more cost effective once you get past a few maps, especially for pilots who have to purchase the printed charts anyway (for practical reasons, if not legally). Also see
http://www.gpsvehiclenavigation.com/GPS/review-e800.php
The problem with any PDA nav solution, though, is that they are not rugged enough. The touch screens need to be only ... erm.... touched and it selects some undesirable function
I think it makes a great backup GPS, especially for passengers to look at (they might also prefer to see road or general topo maps) but I would never use it as my primary GPS.