I bought a refurbished, discontinued Garmin Streetpilot III off of Amazon.com for $400. It has paid for itself many times over, in fuel saved and my sanity. As was mentioned, get the newest one you can ( or can afford). I believe the database in mine is about 3 years old, lots of new stuff was built in three years.
The GPS isn't fool-proof, but it's pretty accurate 90% of the time ( more remote areas tend to be less accurate). If the programmers of the GPS (or map makers) were given bad information, it will find its way into the GPS. Example: "Anytown Dr. becomes "Anytown Rd." simply by reversing two letters. Or a street winds up with three different names within a few miles within the same Zip Code or city.
I do mostly local, so I keep a local street atlas and some back-up "real" maps in case. But knock on wood, the GPS has been good. Plus, many of them have a trip computer function. It keeps track of your average speed, max speed, miles driven, time stopped,etc.