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It says something like 'Off route' on the screen and has arrows pointing you back in the direction you need to go to get back onto the route. The arrows turn red the further away you get from the route. It's nothing better than 'OK', but what's helpful is you can see roads/paths that will get you back on track, so it's not difficult to do that. Just doesn't give you a line to follow. Future firmware updates may change that, I'm told, but for my purposes it was absolutely fine.
Actually, one more gripe (probably more my fault) was that the gradient screen doesn't really give you a great idea of where to go. It shows how far you've gone up the climb and the level of gradient, but when I was climbing out of Amboise and the 'climb' consisted of more than one road, I didn't know where to go. However, it seemed to then give me some indications on the next hill I went up, so maybe it was just me being dense the first time round.
That sounds great, how does it show that you are off the route and does it show you a straight line to where you can re-join the route?