In reply to Welsh Kate:
I've used it for a few years now, but have not used alternatives apart from Trace de Trail. I never print out maps.
I routinely use it to plot new ideas for routes, and then sometimes I save and download these plus a map of the area to my phone. I flip between the 1:25k map and the aerial view, using whichever shows best where the path goes (usually the map). A fancier watch than mine that showed just enough to tell me whether I was on route would be more useful for me more often, but, without that, I have found it incredibly handy when somewhere unfamiliar to have the OS app show me a map, my route, where I am and which way I am pointing.
Sometimes, though, the app tells me I haven't downloaded the map when I am pretty sure I have. I would also like to be able to display multiple routes on a map.