I recently spent a week on holiday in Cornwall, and on the way down, decided to see if the navigation build into the android version of google maps on my old Motorola Milestone was a viable sat-nav replacement.
After sending me mostly the correct way through London, I then ignored it when it told me to head for the M4, as my chosen route was the M3/A303. Turns out that this was a mistake, as the navigation doesn't appear to re-calculate routes particularly well. In fact, all the way to Salisbury I was being told to turn off the A303 and head for the M4. Which starts to grind a little bit when it's every single junction.
It grinds even more when you've made the sensible choice/rookie mistake of mounting the device as far down the windscreen as it would fit, so it only blocked my view of the bonnet. This also meant that the milestone was out of my reach, so I couldn't turn the bloody thing off. Fortunately, due to the crappy car cigarette lighter to micro-USB adapter that was shipped with my crappy el-cheapo in-car phone holder, which needed to be held down in the lighter socket to get any power, the phone ran out of juice, and I was allowed to continue along my chosen route of the A303 without being sent massively out of my way to a motorway I didn't want to use.
Also, what sort of new car comes without an ipod/iphone dock, or at least a 3.5mm stereo line-in, and a USB socket for charging? A VW Golf Bluemotion that a hire car company buys, clearly at the very basic end of the basic specifications. Not even stereo controls on the steering wheel? Poor. On the plus side, bloody good fuel consumption. I wasn't hanging around, enjoying the torque of the 1.6l diesel, and using plenty of air-con, but I still managed to drive a long way down Cornwall, spend a couple of days driving around, and drive back to London before I needed to fill up. Seven hundred and fifty-ish miles from a 55 litre tank, which cost around £75 to fill up at this weekend's prices. Or around 10 pence per mile. I've got no idea how this stacks up against electric vehicles, especially if you have a few kilowatt hours worth of solar panels on your roof, but it seems pretty good to me.
Anyway, if you want a sat-nav to use in a car, buy a sat-nav. If you're in a real pinch and have an android phone? It'll work, and is quite impressive for a freebie add-on to a mobile phone, but I certainly wouldn't use one on a day-to-day basis.
Mind you, I wouldn't use a Tom Tom on a daily basis. I'm a bloke. I like to read maps.
And you certainly wouldn't find me asking for directions ;-)