My 800 with 1:50k OS mapping is easily my best cycling product.
I find the screen fine to read, and it’s hugely better than any smartphone screen when outside. The resolution isn’t as sharp as a smartphone, but it’s absolutely fine for its intended use.
Making up rides on the move is tricky, simply for the lack of screen real estate, but it’s for following pre-defined routes you’ve planned at home (using the OS maps from the device on your computer using BaseCamp). It’s perfect for figuring out where you are in tricky multi-path areas.
I love it for road riding too, simply as OS maps for me are head and shoulders above any other form of mapping. It’s great to be able to ‘see’ stuff around you that you don’t get on any other maps.