Selecting the roads with most bends is just a gimmick, in my opinion, I'm more than capable of doing that myself with a decent map and Mapsource.
I'm an advocate of planning my routes from a map and using mapsource to then send my planned route to my Zumo.
But....don't poo poo winding roads on the Tom Tom. I've got the app on my iphone and have used it on a few occasions to get from A to B, found some great roads I'd never have normally travelled along, usually avoiding the car infested trunk/main roads. No it's not as good as something you've planned but for when you can't be arsed it's not too bad