The c340 provides turn by turn instructions and has enhanced voice commands, i.e., it tells you not only when to turn, but the name of the street as well. You can select from several female or male TTS (text to speech) voices. I've chosen Australian English Karen, hence the choice of name. The c340 also has a database of points of interest, restaurants, gas stations, ATMs, etc and a few strokes on the touch sensitive screen will have you on your way to what you need in a few seconds. The interface is completely intuitive and everything is ready to go right out of the box.
The only drawback I've found with the unit is that the map database is probably several years out of date, at least for the area where I live, in a fast growing area east of Phoenix proper. The unit knows nothing of a new surface arterial near my house that has been open for at least 3 years, since I moved here, and a new freeway that has now been open for one year. The result is that if I want the unit to guide me from home to my destination using those roads, it will be busy announcing constant recalculations of route until it gets back into the known world. I suspect this will be an issue with all Garmin GPS units as they share the same map database. Other than that localized glitch, I'd highly recommend this unit. Garmin apparently has a new version of the map software coming out later this year.
UPDATE 11/25/2007: Above I complained about the map database being out of date and was fearing that I would have to pay for an expensive map update. I just learned that I could get updated map software, the 2008 version of City Navigator for free ( because I registered with Garmin). It took me 30 seconds to order online and it will be shipped to me. The vast tracts of Terra Incognita that started less than a mile from my house are now cognita, er, or something like that.