In reply to ow arm:
I have largely changed to Harveys Superwalker 1;25000 maps in recent years. The main reason for this is that the Harveys 'generally' provide more information that is useful to walkers than OS. In particular the designation of paths/tracks on-the-ground, and intermittent paths/tracks on-the-ground is better than OS. In my experience, indication of walls/fences is also better on Harveys. The 15m contour interval compared to 10m on OS also leads to less dense contouring on the map, making other features more visible, but does need you to make the mental note if used to OS. Some sheets, such as my home turf of Dartmoor are in 1:40000 scale, which again requires some mental retraining if you're used to OS 1:25000 & 1:50000, but the grid squares are still 1km, so its quite easy and Harveys sell a separate roamer inc 1:40000 for £1.
The flip side of the Harvey v. OS debate, and again relevant in areas such as Dartmoor, is that OS maps tend to have more historical data. I can think of several small stone rows, cairns, hut circles etc on Dartmoor that are described on OS maps, but absent from Harveys, and its quite useful to know where they are when you're fogged in.
On balance though, both series are excellent maps and cost less than a tenner each, so why not carry one of each.
N