In reply to Lynsety:
Got the OS maps pre-loaded on mine - I think it was something like an extra £50 as a package, but over £100 as a later add-on. You can get open source maps - either free or for around £10 - do a search on OSMcycle maps I think.
As has been said, the device is awkward to use for on-the spot navigation due to it size and slow speed - if I get really lost I tend to get my phone out and use the Motion-x app, which itself uses open source maps. But most of the time it fine for simply confirming where I am.
So my advice to you, given that you haven't got the OS maps preloaded - is give the open source maps a try.