We use the Etrex, too. Get one where you can plug in an external aerial if you can afford it as the basic models do lose the plot a bit. I know ours works better when I'm using it as I'm slower than my husband, so doesn't lose lock as often. It goes in the camelbak or in the bag on my KDX.
We use it for seeing where our trails go but also for making new ones. Just mark a waypoint & head towards there, instead of relying on you keeping an eye on which direction you want to go to.
As for using it to mark an injured person, I never, ever, ever want to do that. However, we have told the ambulance staff that we have one with us if they do need to figure out where a lifeflight has to go. (We organise events and here in NZ, for insurance purposes we have to have trained medical staff onhand and to be honest, those of us running the events prefer it not to be us).
As for mapping software, we use GPS Trackmaker -
www.gpstm.com
We download to the computer & can alter the tracks if we want, paste a picture behind it.
Hope that helps
Michelle