For the 305, you plot the route first (either on mapping software or the web, e.g.
www.mapmyride.com) and then load it to the Edge as a track or course (I use tracks). You then simply follow the 'bread crumbs' display. There's no map as such, or turn indicators... you just simply follow the direction shown on the Edge. I think you can mark turns etc as waypoints, or whatever, but I've never bothered as there's a limit to how many you can store per track and, in all honesty, I've never needed them.
You can have a situation where there might be an intersection where a number of roads go off in similar directions and it's not obvious which one to take but, in practice and ime, it doesn't happen often enough to warrant calling it a problem.
Edit: the 705 is true sat nav as you have it in cars etc. Route planning on the fly. That is, you can key in a post code, say, on the unit and it will direct you there. The 305 knows nothing of maps or roads etc... it simply directs you according to the route you have plotted beforehand and loaded to it.
I bought my 305 before the 705 came out but, even so, I have no need for the extra functionality of the 705 and will happily stay with a 305.