On my phone I have a satnav which has maps stored on it, therefore doesn't require cellular coverage and doesn't cost me a penny after installation - this Co Pilot live application is even from a "real" sat nav manufacturer! A single payment of £20 was all that was required IIRC.
Oh, and Google caches your route so if you follow the instructions it doesn't matter if you lose cellular coverage.
The only issues I've had are:-
1) The pathetic volume level on my Xperia X10 - can't hear the directions unless I mute the radio (fortunately I generally know my route so this is a minor issue, and when I am in an unfamiliar area of the main roads I tend to lower the volume anyway)
2) When you end up actually needing it (Wakefield town centre signage I'm looking at you) sods law dictates you'll be on the phone (via the bluetooth system) and unable to see the sat nav screen or get the audio from it!
3) Androids "Don't Lock the screen while charging" setting is awesome as it means the satnav stays on screen, but once the battery is fully charged it stops charging and starts locking the screen - argh!!! (I wish it was smart enough to not lock the screen when the charger is connected). Fortunately it does pop up on screen when you are 1km from the next turning and stays on screen until you more than 1km from the next turning.
Even with those issues for a reactively light user like me mobile phone based sat nav works fine.