If it weren't for the patchy performance when uploading rides to Strava, I love my 810. I find the screen plenty clear enough to navigate by and because it's touch screen, it's easy to hit the zoomin/out arrows when riding and you want a bit more local detail.
I've tried one decent MTB ride, with a map a friend sent, following footpaths/byways and I had no trouble working out where I should be going.
I'm not sure what the 1000/1000 Explorer offer extra, but when I was purchasing they weren't on my radar - I was making a 520/810 choice and the touch screen won.
Russell