I've had my Exec for a month now and used the Nav quite a bit. It's only 'failed' me once, and that in fairness was trying to find a tiny hamlet in Scotland without postcode info. I still found it though - the moral of the story being that however flash your satnav always carry a map....
Back to the Exec. I agree that there is a fair bit of twiddling and pressing involved in inputting a destination but I don't find the time and effort required much different to the touchscreen in my wife's Honda. And of course, not being a touchscreen, the Lexus screen stays nice and clear.
I've not tried the Premium system but I certainly don't feel short-changed.