Re points below:

Dead reckoning, nice, but I have not found it necessary on 99% of drives. You are always going to get a little more oomph in a mounted system, but I actually value the portability more now that I have it. Example, when my windshield was rocked, I had to drop the car off for the day, so I tossed the Garmin in the rental to do my rounds.

Sound - I have no problem hearing the Garmin with the radio on. I do find the female voice a little clearer.