I review (editorially) a ton of BlackBerry (and other) products, including the next generations of navigation programs. I've also reviewed a ton of handheld & dashboard navigators.
Hands down, the server-based approach is smarter and gives you significantly more accurate maps.
But you are wrong to believe that this approach relies on an unbroken cell connection. For the major players (Garmin, TeleNav, AT&T, MapQuest, etc.), maps for the entire travel corridor all get downloaded into the handset at the beginning of each trip - they're not at all dependent on the cell signal never disappearing.
These also get ongoing en route updates of traffic hazards and other travel factors en route - but the updates don't need to be constant to be valid - if one is supposed to happen when you have no signal, it just waits until you do.
In my opinion, the only advantage to a dashboard navigator is a bigger screen.
Frankly, I'd love to have a car with half a dozen BlackBerry 9000 Bolds on the dashboard - one running Traffic Vizzion to give me traffic camera video of the road ahead - one running Trapster to warn me of speed traps - one running a navigator - one to let me do 2-way video calls (coming later this year from third parties) - one for normal phone calls - and one to read me my e-mail. Heck, I'd probably have one of them playing Sinatra in the background.
The idea of storing maps may seem attractive because it's possible, but I've tried it both ways & I like served-fresh-right-now server-based maps a lot better.
And don't get me started on VZ Navigator - I tried it when it was new, would not track without a constant connection, burned your minutes while tracking and had up-front and monthly fees besides. The Verizon PR
guy is a lovely fellow I find likable, but I wouldn't raise a finger to stop the company from disappearing - I've seen way too much of what I believe to be dishonesty and gouging to have any love or respect for them.
I like AT&T - even though "more bars in more places" is what their local coverage drops often compel me to drive to.