There was a recent NYT article comparing three of these things. Unfortunately, they want money to see the article now, so I can't bring it up easily.
In addition to the Garmin, there's also the
Magellan, also having all of the U.S. and Canada in a device you can carry with you when you travel. A friend of mine has one and says he loves it. I'm personally not sure whether I'd rather have a portable GPS unit like these, or whether I'd rather have something built into the car. I imagine the portable is more useful, particularly if you fly places and need help navigating unfamiliar neighborhoods.
The only thing like this I've ever actually tried to use is Hertz's Neverlost system (an older generation Magellan system). When it really mattered (trying to find a friend's apartment in Hoboken), it steered me the wrong way on the Turnpike and nearly took me through the Holland tunnel into Manhattan. Once I realized I was screwed, I was able to get on some local roads and make a phone call, getting normal, traditional directions. I can only hope the newer nav systems work better.