My Explorer Scouts use Highlander ones like this
http://cgi.ebay.co.uk/HIGHLANDER-DE...hZ004QQcategoryZ72895QQtcZphotoQQcmdZViewItem and at that price I think they are good value.
You can get 6 sections of an OS map visible at once, although mine tend to have their route plan (A4 sheet) in there too which works nicely.
There is a pocket at the back for a notebook or second OS map covered by a velcro flap and a holder for 5 pens! (Who ever needs 5 pens?)
However there is no compass pocket so I remove the stiching between the three middle pen holders and its just the right size for most baseplate compasses
If you go that rout then I recommend that you check the width of the pen holders first as they are not completely consitent and on one I had to merge three holders off to one side in order to get the compass in, either way its a 5 minute conversion, if that
See later post - these things let me down badly on an autumn expedition, I have replaced them with Ortleibs.
By the way we remove OS maps from the cardboard cover (carefully pull them off) as it makes it easier to fold into a case. We still keep the covers, and stick an A5 enevelope inside so that you can put the map back, fold the top over and they can then sit nicely in a box with the spines sticking up for easy access. They sem to last longer that way too!
I can bring one back from the HQ next week and photograph it if that helps.