Before you buy a GPS, think about the options that will best suit your needs.
Good navigation: No GPS is perfect, but the directions for local roads can vary. Research reviews of a unit's capabilities at Web sites like cnet.com and amazon.com.
A 3.5-inch screen: Most systems have 3.5-inch screens (measured diagonally), a good trade-off between convenience and legibility. (Go larger if you have weak eyes.) Be sure you can see the screen from all angles.
Text-to-speech capability: This increasingly common feature guides you with specific names ("Turn left on Elm Road.") instead of a generic "Turn left at intersection."
Easy-to-enter information: Many units make it convenient to put in a destination by trying to complete your words for you as you type. Some higher-end models even use voice commands.
The right maps: Find out what maps come with the system. On cheaper units you may get only a basic map with major roadways, then you'll have to pay for expensive add-ons. Also make sure you can update the unit at low or no cost as newer maps and system revisions are released.
Internal rechargeable battery: Because you can use a portable GPS away from your car (say, for hiking), get a good battery. It should work for several hours per charge, giving you directions and peace of mind.
Points of interest: Most units can guide you to a nearby gas station or ATM. Get one with plenty of these "points of interest" (POIs) to simplify your life, especially when traveling to and within lesser-known destinations. Look for models that let you program personal POIs.