If you spend much time driving in unfamiliar territory, especially if you rent cars in big cities, a good portable GPS makes an amazing difference. And the Garmin 255W is the best one I could find in the $250 - $350 range.
First this thing just plain works. I haven't had any issues with mine. It was easy to set up and figure out right out of the box, and it's easy to use. It never has had trouble getting sufficient signal unlike earlier and cheaper models.
I chose the 255W for several reasons: It uses some of the newest and best maps available. It has one of the more usable touch screens for entering destinations. It's fast to acquire satellites. And Garmin almost always comes out on top in reviews--especially in routing.