Garmin's routable mapping is the single best and worst thing on the edge 800.
Best - easy to navigate on road rides, just enter the postcode of a cafe ~30miles away and pedal. Shortest distance algorythm means it's generaly using smaller roads.
Worst - rather than re-calculate if you go wrong, get diverted, pick another route, etc, it seems to try and put you back on track. But will never U-turn. So you can add 20mile loops to a ride just by missing a junction. A recent example was riding from Reading to a cafe near didcot (north west), the suggested route was tot ake themain road west out of reading which is horrible, so I headed north expecting it to recalculate and send me cross country/west once I was out of town. It's solution (repeated through about 6 recalculations each time i ignored it) was to turn east, and try and do a full circle of Reading coming back in from the East.
It's Ok for navigating to places in fairly straight lines, but if you want to ignore it it's best to re-set the route once your past whatever it is you're riding round rather than through.