I have just bought FlyMap installed on its own dedicated (roadangel) unit. It is very good. Like IO540 I thought these units would be too small to run the digitised half mil effectively but changed my mind once I saw it in action. The benefit of the dedicated unit is that it is more robust, smaller and has a longer battery life that most PDAs. Navigating through the software is very straight forward using a touch screen.
It comes with the software to plan your route on the PC, check NOTAMs and then down load the route. Unfortunately the unit is not compatible with the likes of NavBox. However you can also get the quarter mil charts and next year I shall invest in a set of the Jepperson French charts for a trip to France.
I also wanted something I could easily carry in a Christen Eagle and the unit can be strapped to your knee or wrist, which saves trying to find somewhere to mount it. In all an excellent though quite expensive backup navigation tool.