I have developed two separate mkgmap styles. One is used to create maps for my GPS unit (and is called CF_GPS); the other is used to create maps for Mapsource (and is called CF_Mapsource). The reason I use two different styles is because my GPS unit is incapable of correctly displaying any roads that have borders (the so-called "spaghetti problem": see this, this and this). Thus CF_Mapsource creates pretty maps with nicely drawn roads (with borders) whereas CF_GPS creates less beautiful maps for my GPS, but at least they are usable.
The following screenshots are taken from Mapsource and thus demonstrate the CF_Mapsource style in action.