Actually, it'll always take me back to my custom route if I leave it for a restaurant, to explore, or whatever. I can even cancel the route and start it again and even if I'm in the middle of it, it'll do things like ask me if I want to navigate from the beginning or from where I am and then tell me how to get back on track.
I certainly don't know how the Zumo's do it, however, so there may be some significant things I'm missing with the Nuvi. And it may be that my Nuvi does things that others don't ... just no idea. But as far as routes go, there's nothing I've not been able to do so far, and I love being able to use Google Maps to plot any route I want, save it off, edit it again, share it, whatever. It even takes me through roads it doesn't know about (and Google does), so that works great too. Love it.