There's a free app & a paid app (£10).
The free app is for iphone use only, you pay for & download maps directly to your iphone, you can transfer gpx data via email or through websites, but you wont be able to use the same maps on your PC.
The paid app allows you to use the same maps on your PC & you can transfer MM files using wifi.
The PC software is free, you pay for the maps. The CAA charts are £20 each on DVD, I think the downloadable charts are cheaper. What you're paying for is a licence to use the charts on one computer & one mobile device; you can migrate the licence to new hardware when you upgrade your PC or phone simply by going online & signing in to your MM account - no charge.
More info:
youtube
MM
MM support forum
You can plan routes on the PC & transfer them to the iphone, you can also plan routes on the iphone. You can print a route card off the PC. I think that MM latest iphone app is purely GPS, no 'A'.
I used MM on a WM5 smartphone for 5 years without any problems. I've just gone Android & will buy the Android app when it comes out.
It's the OS that give MM their monopoly, not the CAA.