A battle for market share among the big three manufacturers of navigation devices — Garmin, Magellan and TomTom — has forced down prices of portable GPS units over the past year. Plus, consumers are responding to an increased ease of use: The daunting menus that once characterized the devices have been replaced by simple touch-screen icons.

For those out of the geosynchronous loop, the global positioning system is a group of satellites that allows those with receivers to pinpoint their exact location on the planet, plus their speed and direction, and tell them how to get where they want to go.

To that end, you simply enter your destination on a touch screen, and a small map pops up with your route highlighted and arrows showing where you need to turn. Voice prompts may also tell you, for example, to "turn right after 300 yards."

But the units' voice technology isn't always perfect. Approaching the dreaded Capital Beltway that surrounds Washington, D.C., one device insisted that I needed to make an immediate right. I ended up in a mall parking lot, off the grid. The voice fell silent until I drove to a road it recognized. Another unit's map accurately pointed me to the left, but its voice told me to bear right. Moral: When in doubt, trust the map.

List prices for GPS units bear no relation to reality, and ordering online is usually cheaper than buying from a big-box retailer. Although you may find a cheaper price at another e-tailer, Amazon.com has the best selection.

For basic navigation, consider the TomTom One (recently $142 on Amazon), Garmin Nuvi 200 ($138) and Magellan Maestro 3100 ($130). Should you spend more to get better directions? No, says Fletcher Previn, of GPSmagazine.com. The best-kept secret in the GPS world, says Previn, is that within a particular product line, "the $200 model will choose the exact same route as the $1,000 model."

A basic model can also direct you to nearby gas stations, restaurants, police stations, parking lots and points of interest. You can tell the unit to take you via the shortest or quickest route, to avoid toll roads, or to travel by way of a favorite road.

You'll need to update your maps at least once a year. Garmin's annual updates are $70 for the continental U.S., and Magellan's run about $80. TomTom offers updates by region. For example, the New England/Mid Atlantic map costs $60; the entire North American map is $100.

Robert Frick is a senior editor at Kiplinger's Personal Finance magazine. Send your questions and comments to [email protected]