I have a subscription to the Dutch version of 'Consumer Reports'. They recently tested a handful of dedicated GPS models against a smart phone; the smart phone came in first. It appears Google Maps and the various apps for the smart phone do a better job than the dedicated GPS devices.

If you go that route you'll have to buy a holder for the phone and it would be wise to buy an external power pack as well.

My local bike store tried to interest me in a dedicated device (the Mio), but without doing any research whatsoever thought a smart phone would do the same job but for considerably less money.

I still haven't bought a holder or a power pack and I'm in no hurry to do so. I'm glad that the consumer magazine confirmed what I already thought.