I found the Tom Tom unit to be not as good as the standard Lexus sat nav;
- it is significantly slower to auto re-route if you take a wrong turn
- it is not as vocal, meaning you have to pay more attention to the screen
on several occasions using the Tom Tom, it did not give us vocal indications of when to take turns, which lead to us getting more and more off the route.
on one occasion, it gave us a vocal indication literally exactly as we were at the junction.
its a good unit - I would recommend getting one if you don't have factory sat nav, but I don't think it is as good as the standard nav I had in my IS, or currently have in my GS
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I have the iPaq with Tomtom 3 and its quick at rerouting (less than 200 yrds @30mph it will tell me I need to turn around. Never encountered any problems with vocal. When using the sat nav.. i disable all the sleep modes on iPaq to avoid any of the situations you mention Matt... Might be worth looking at the settings on your ipaq Matt