My Garmin has been spot on with ETA's, but that may be the autobahn factor, we're not such snails like in the US.
No navigator can account for traffic lights or unexpected stops, if Sununu says that his navman did that he's just lying.
You can add a detour to your current route if needed, just hit the menu button while navigating and select detour.
I've had several Garmin GPSs, some had an option to avoid certain roads or areas but the interface on those was a lot more difficult, I guess Garmin wants to keep the nüvi simple.

John AH, the TomTom One is also half the GPS.
Compare it to a Garmin Streetpilot 330 or a nüvi 350 for the same capabilities (plus the nüvi is still smaller)

The latest Garmins can save their tracklog onto the datacard (this one will probably be able to do that too), thereby eliminating any restrictions.

nemi: The Garmin's and this new one are waterproof.

That's exactly it, the installed GPS costs 5 to 10 times more and you're stuck with it for the entire life of your car, a PND can keep up with the latest trends etc. The Garmin n?can have traffic information fed to it via an FM add-on, try adding that to a built in system, other Garmin PND's can get XM Satellite traffic and radio.
The possibility to upgrade is much bigger and less expensive, PND's are the way to go, plus you only have to make one purchase for multiple cars, you can take it on vacation (even use it in the plane) ... yes I love my portable GPS ;-)

Garmin Streetpilot 2720 (or 2730, XM weather yeah!)

TV's still come with knobs you know, even my LG 42" LCD has knobs to change channels, volume, menu and on/off.
What about just telling them not to watch and they obey, how weird would that be huh?