The OS maps are lovely, and interesting for some of the subtle detail they include. I noticed recently (while using an OS map as a background raster for some much more detailed data) that the roads on the 1:25000 and 1:50000 scale maps are drawn with accurate widths derived from the more detailed survey data.
The source data the maps are produced from is also pleasingly detailed, they sent man to the remote part of Scotland where my dad lives to properly survey all new houses. Openstreetmap has that sort of data for central London, but not for anywhere like the whole country.
Bing Maps UK has OS Explorer and Landranger coverage as an option. It is extremely impressive, as the sheets (which are drawn to the UK National Grid) are subtly distorted to the projection used by the Bing mapping (for example the grid lines are not all straight up) and routes can be calculated against the layers - even though they're raster data.