I have a Garmin StreetPilot 2720 and couldn't live without it as a realtor! Best money I've ever spent!
I originally had one of the smaller Garmin units (can't remember the name of it), but exchanged it for the StreetPilot.
Even though the 2720 was twice the cost, I couldn't read the screen with polarized sunglasses on the smaller one, and with my light-colored eyes, I have to wear sunglasses when I drive.