This map helped me to find my way back into Stornoway after I was shipwrecked by the Butt of Lewis lighthouse after a terrible illness befell all my crew-members after I shot some seagull or something on our voyage to Iceland. Anyway, long story short, they all got quite ill and didn't make it and I was washed up on the rocks by port stoth after using this map to fashion a makeshift raft. I was further able to use the handy key to determine my location and walk my way to the nearest small town of Eoropie and hitch a lift with two nice Australian people into Stornoway.
It has everything you might look for in a map, which is really the point of ordnance survey. Bear in mind that these maps do not contain information about depths at sea, lighthouse beam angles or locations of reefs or rocks, and as such are unsuitable for nautical navigation.
Most useful when in conjunction with the more detailed 1:25k series, which are so detailed it is difficult to see the wider picture. North Lewis is possibly the most barren of the islands and so this map is vital to see how the settlements on the west coast link with Stornoway, which is where most visitors will either enter or leave the islands.