OK, I promised an update on the TomTom App, and here it is:
We have just used it for driving around New York (though not Manhattan), including JFK to/from Brooklyn and Long Island, and then on to Cape Cod for a few days.
This was the USA and Canada App, downloaded into my iPhone 4S; cost £39.99.
It was absolutely great.
As it has the standard TomTom satnav features, there's no need to go into that part very deeply, so I'll concentrate on how well it worked on the phone.
Pros:
[*]Extremely clear screen; the iPhone is a better screen than the standard TomTom one, I reckon, and it made good use of that. We had it propped up in front of the passenger's seat, and the important info at the bottom (distance to next 'event', direction symbol, etc) were easy to see from the driver's side;
[*]Very accurate GPS. Like screen clarity, I guess this is down to the quality of the phone's system.
[*]Fast GPS lock-on; handled short disruption (eg tunnel) very well;
[*]Clear voice instructions, though not as good as the 'native' device, which is exceptional, in my opinion. There is a choice of voices.
Cons (none serious):
[*]The iPhone screen is too small to view the map whilst driving, though it is very clear. An iPad may be better, but we didn't miss it;
[*]The touch screen was a real nuisance at times. A single tap switches from guidance view to menu, and it's too easy to do this by accident whilst doing the 2-finger zoom. Another single tap restores the map, though.
[*]Again, zooming the guidance map was tricky; it was too easy to overdo it. Switching to Browse Map worked better.
[*]The guidance was decidedly dodgy on occasion. Why is it that all satnavs seem to have obsessions about certain roads? My in-car one here at home is obsessed with the M25 and will take us miles out of our way to use it. This TomTom app was equally obsessed with avoiding Route 28 on Cape Cod, even though our hotel was actually on it. We always use a road atlas as a reality check before setting off. On the other hand, it navigated us immaculately through the maze of freeways and parkways around Brooklyn, Queens and the Bronx. There is an option to avoid road tolls if you want.
[*]Unless you are prepared to pay high roaming data costs (we weren't), or have a tariff which includes data, you must turn data roaming off on the phone. This means you can't get on-the-minute traffic data. We didn't miss it, as we don't normally use it over here, either. When you load it, the app puts up a warning "data services off" or something like that. Simply accept and carry on.
Other points:
[*]I was mistaken earlier; the iPhone's Airplane mode does turn off the GPS. I used it all week in normal mode but, as I say above, with data roaming off.
[*]GPS is a battery-killer, but I bought this from Amazon before I left
Dual USB 2A 10W (fast) Heavy Duty Output Car Charger
We kept the phone plugged in all day, and it charged it up to 100% whilst navigating. The 2nd USB slot was also useful for other gadgets in parallel.
All in all, amongst the best value £40 I've ever spent. To my mind, there is little point in buying a dedicated device if it works this well on a modern phone. Note that all the above is based on iPhone 4S/iOS 6. Other reviews imply it works just as well on Android, but I have no direct knowledge of that.
Hope this is useful. I've posted it here in case others are interested. If you have any more questions, please ask 'em.