I've used a Garmin eTrex for years. It only comes out of my pack in poor weather and probably isn't suitable for driving. I don't have SatNav or GPS in the car but my wife does and I only use it for traffic alerts if we go on long journeys.
I got an iPhone recently and found an app for that which I used in the Lakes last weekend (just as a comparison) and that was pretty accurate (see the topic I just wrote here
http://www.walkingforum.co.uk/index.php/topic,2034.0.html [nofollow]).
To be honest, though, I wouldn't use a phone as my fix-all solution for a GPS in the hills. Phone batteries should be best conserved for emergencies and if you're using your mobile/GPS all the time to navigate with, I'd rethink your navigation strategy on the hills. I don't think there is any substitute for good map and compass work with a GPS on standby as a tool to confirm your position.
Oh, and a GPS that's good in the car may not necessarily be good on the hills!
macbloke