I have used a basic Nokia (5230 or 5320 - I can never remember which) with a Nokia charger on a hub dymano and Viewranger app. The 'rubber band' holder with the Nokia charger is good at holding the phone.
Issues are:
1. Water resistance (you can put the phone in a clear dry-bag)
2. Visibility - very difficult to see the display in bright sun.
3. Using the touch-screen while on the move - small drop-down menus while coping with vibration & cycling gloves.
4. Display is quite small and I need glasses to see the display.
Viewranger did a free download of GB streetmaps a while ago - you can also use other mapping with it or online.
The Viewranger app is very comprehensive and complicated, probably over-complicated for use on a ride- putting a route in is not at all straight-forward.
Nokia's Ovi-maps is a good, easy to use Sat-nav that is ideal if you are lost and want 'a' way to get home. It dioes like to use motorways if anywhere near, though.
Again the maps are on the phone, but the phone goes online to find the address/postcode/destination. It is not possible to import a gps to Ovi-maps or plan a route on anything other than the Ovi-maps website.