The Ordnance Survey is one of the few institutions (perhaps the only one) that still makes one proud to be British (another thread in there?). I agree that its mapping standard is hardly equalled for clarity and ease of use by any other European country, though the Swiss official topo maps are works of art, especially in the mountain areas, with their beautiful hill shading and colouring.
The main problem I find with OS is that the orange-covered Explorer maps are unwieldy: too big to be used in the field on a windy day. The old green Pathfinders, of which I have about 750 and buy more when I see them in charity shops (and can remember if I've already got them or not, which I rarely can), were just the right size. What I can enthusiastically recommend are the Yellow Walk maps (
www.yellowpublications.co.uk) which take a smaller area, e.g. Arnside and Silverdale, and enlarge it to 1:16,000. These maps fold to postcard size (and about 8 or 9 of them fit very snugly into the boxes that Batchelor's packet soups come in). They can easily be used in the field, without blowing all over the place, and are showerproof but not totally waterproof. I find that I can read the print on them without my glasses, which I can't do with Explorers. They are ideal for areas like Calderdale which have a very high density of footpaths per square mile. The Hebden Bridge-Todmorden area is one where you really need something larger than the Explorer.
The bane of any paper map is that the place you want tends to be on the join/fold. Kettlewell in the Yorkshire Dales is a case in point. But there is an AA walkers map (no. 7, Central Yorkshire Dales)which is just a reprint of the OS map with different sheet boundaries, and that solves that problem. I found that when I wanted to walk in the Whaley Bridge area, it was awkwardly on the join of 2 or 3 maps, so I got a special sheet printed by an outfit in Hope, Derbyshire, which will do you a showerproof map centred on any point you choose. I opted for Whaley Bridge station. Nowadays I've got the area covered by Yellow Maps, so I don't use it much.
Good sources for second-hand OS maps in the northwest are Carnforth Bookshop and Bookcase in Carlisle; the bookshop in the old chapel in Sedbergh is also good as is George Kelsall at Littleborough (in the house at the back). But I still miss the Manchester branch of Stanfords. I completed my set of the last 1-inch to the mile maps and am well on the way to completing the Landrangers for England and Wales and the more urban areas of Scotland. Nowadays, I'm told, the Explorers outsell the Landrangers, which is the reverse of what used to be the case.