I’ve been pondering one too… the conclusions I’ve come to are;
Certainly doesn’t replace the map – as you say, so small a screen that it makes that hard.
But work out an intended route beforehand and load in, then just use the unit to make sure you stay on course – excellent.
Being able to download someone elses route and follow it – excellent.
taking down as much data as you want to (cad, hrm, etc) if you are interested – excellent.
as a ride log (which you may/may not find motivating/useful) – excellent.
Having said that, 500 is cheaper and you can do these same things, bar an actual map. Bryton units are cheaper again. Really depends on how much you think you’d be using it as a map. personally, I’d not; just take a paper one (and compass!) in the bag with you if somewhere you don’t know etc