The screen is inevitably a bit small, so if the map has a lot of detail on it, I suspect that if you zoomed in enough to see it all, you won't have much of the map on the screen!
I find the 1:50000 perfectly adequate for cycling, and it only (!) cost £100 or so for the bottom half of the UK. The same area in 1:25000 would be something like £2000.
I do also have the London A-Z which is 1:14000, but where there are lots of roads, it's a lot easier to see things at this scale, with the 1:50000 mapping, sometimes you're not even sure which of two roads you are on, since errors in the GPS/mapping can place the track smack bang between two roads! The 1:14000 seems to be better for this sort of issue.
If you give me the location of a bridleway, I can take a couple of shots of the screen, zoomed in and out as much as is usable, that ought to give you some sort of idea how well it works. As perpetual dan said, it's essentially identical to the OS 1:50000 Landranger maps.
I have two Li-Ion battery packs for mine, and that will keep me going for an entire FNRttC, even with quite extensive use of the screen (and I think I have it set to stay on for five minutes).