I bought my wife a Garmin C340 last year and she loves it.
Most units have all of North America loaded from the factory. Most have USB capabilities to download newer map information. Most offer 2D and 3D views.
The main difference between the cheaper and more expensive systems is in the turn by turn information given. The less expensive unit will say for example...
"in 200 feet turn right" or "in 200 feet bear left onto exit ramp"
where as the more expensive units will say for example...
"in 200 feet turn right onto Main Street" or "in 200 feet bear left onto exit ramp number 29, I-91, downtown Hartford"
The more information given, the better my wife liked it.
If you can get by with simple directions great, but sometimes at complicated intersections some extra confirmation is helpful. Also, don't let the GPS unit override your common sense. GPS units generally have two algorythms for computing your travel route, fastest route (tries to maximize highway use), and shortest route (shortest travel distance from point A to B). The route calculated might not be your preference or may cause you to back track out of your intended route to reach a highway quicker. If you know a better route, disregard the GPS unit's instructions until you reach your intended route (the GPS will continue to recalculate new routes, and eventually it will figure out which way you intend to go), or learn how to add way points to your travel route with your system so you can program your route via any highway, city, or street you want to travel.
The GPS is a great tool, I see them becoming a standard item in new cars more often soon.