I have to laugh every time people say "But I can do this with my phone!" I prefer to have the dedicated unit, because if you're using your phone as a GPS and get a phone call (and use your handsfree, of course), you lose your GPS for the duration of the call.
I've used my smartphone's GPS function in a pinch -- usually when I've forgotten my TomTom at home -- but I really prefer having the dedicated GPS unit.
Besides, my phone's usually streaming Pandora when I'm driving (I connect it to my car's cassette player with an adapter), so I can't have it stream the tunes *and* give me directions at the same time.
Again, Woot, if it's got lifetime traffic and maps, this is a definite yes!