Hi, here's a link to our page in the GPS Buying Guide about some of the features these devices have:

We've found that both the TomTom and Garmin devices, even at the low end, have good interfaces. You should look for one of these brands, and try to get the biggest screen you can afford, as the size of the LCD makes a big difference in usability. It also helps safety, as it is easier to see the route guidance instructions at a glance on a bigger screen, as opposed to a smaller screen where you might have to look at it a little longer.