I have a satmap - it is excellent - best toy I have bought for the bike.
I run it with a 1:25000 map of a 70km squared sector that covers all my riding options (Tunnel Hill, Surrey Hills, Swinley Forest), which was about £80 or so.
I have uploaded loads of gpx files sourced from the web (bikely, etc) and recorded loads more from rides with groups like the Muddy Moles, MTB Britain, Diary of a Mountain Biker.
You can also trace the route you want to follow by dropping waypoints with the joystick control, or set a quick goto point to enable you to cycle to some location.
You can set it to orientate the map to the direction you are riding and also mark your tracks with red dots, which is useful when used in the trail orientation mode.
You activate the route you are going to follow and it is shown as a light brown line on the map with direction arrows (which you can reverse) and, if you set the res. to about 1:3000, you can ride at pretty much full speed whilst following the route.
You can also set it to display all inactive routes as a grey lines, so you are aware of all the other trail options in the area (useful in closely packed areas like Swinley forest).
As it just shows an OS map it is easy to break out of a route early and find your way home (even in the dark if your lights fail).
I also use it in the car with a UK 1:5000k map as I am tired of car type satnavs sending me up unsuitable roads, etc. I either trace the route or just drop a goto point and then zoom in and out as required to see where I want to go.
The company is British (Leatherhead) and was started by an ex-RAF pilot I believe so he probably has a good idea about navigation.