TomTom is the latest manufacturer to jump into the growing "shirt-pocket" GPS market with its new TomTom ONE. With an estimated retail price of $499, the TomTom ONE is sure to give the competition a wake-up call.

The TomTom ONE is the company's entry-level product and their first that is designed, like the Garmin nüvi 360 and the Pioneer AVIC-S1, for maximum portability. Weighing just 5.6 ounces and measuring a compact 1.0 by 3.8 by 3.2 inches, it still sports a large 3.5-inch screen that's quite viewable even in bright sunlight. Maps for the United States, Canada, and Guam, based on TeleAtlas's map data, are supplied on a 1GB SD memory card. Like most of the recently introduced GPS products I've reviewed, the TomTom ONE is based on the ultrasensitive SiRF Star III GPS receiver. The SiRF Star III acquires satellites quickly, and the sensitive receiver keeps track of your position in conditions where earlier-generation receivers would have lost signal completely.

The built-in lithium ion battery can be charged though the included DC power cord or a USB cable. A nice feature of the ONE is the mounting bracket. It's one of easiest-to-use brackets I've seen. You merely slide the ONE onto the bracket and stick it to your windshield. The suction cup doesn't have a lever to lock it down to the glass – just pushing it onto the windshield is enough for it to adhere securely. A friction ball-and-socket mount lets you easily adjust the ONE for the best viewing angle. A word of caution—you might want to moisten the suction cup to ensure good adhesion to the windshield.

The user interface on the ONE is basically the same as on all other TomTom products, as are the underlying software and routing engines. The only hardware button is the power on/off switch, located on the top of the unit. All input is done via the touch screen. A tap of the map brings up the first of three main menu screens, each with five choices. As with the TomTom Go 910, the first entry on the main menu is "Navigate to." This menu has a total of ten choices, including Home, Favorite, Addresses, Recent Destinations, and Points of Interest (POI). If you choose to navigate to a POI, a submenu invites you to choose one near your current location, in a city, near your defined home destination, along your planned route, or near your destination. Finding POIs near you is simple: from the map screen it takes just five screen taps. Once you select, for example, restaurants near your current location, the ONE presents you with a list in ascending distance from your location. You can either select a restaurant, or hit the Find key to bring up the keyboard so that you can enter your search for a name.

I found one small annoyance: By default, when you select a POI, the ONE immediately calculates the route to this POI. Sometimes all you want to know is the location and phone number. Other GPS brands show an intermediate screen with the basic information and an option of then calculating the route.

Once the TomTom ONE has finished its calculations, you are presented with a summary screen showing your route on a 2D map. The summary screen also shows your battery condition, your current latitude and longitude, the number of detectable satellites, trip time, and distance to destination. From here, you can either tap Done to start navigating or select Details to see information about your route. The ONE, along with other TomTom products, has the best detailed route information I've seen. You can browse a turn-by-turn list of directions, view maps as images, peruse a map with zooming capabilities, or see a demonstration of your route. You're also able to set the playback speed for the route demo at up to 500 percent of real travel time.

Like the TomTom 910, the ONE has a number of preference settings. By default, you see only three screens, with five choices each. If you're a more advanced user, you can choose to view all preferences, which expands the screen count to seven. You can then set routing preferences for shortest or fastest routes, routes that avoid freeways, walking routes, bicycle routes, or routes that factor in limited-speed roads. I selected to be prompted for route preference for each route. In addition, you can choose the colors of the maps, the POI categories that show up on the display, and the language and the voice directions are spoken in and how loud they sound, to name just a few more options. The ONE also has a slick volume feature that lets you link the unit's volume to car speed—the faster you go, the louder the spoken instructions are.

So you're probably asking, "If the ONE has all of these great features, what's missing when it's compared with the company's top-of-the line GO 910, or the mid-range GO 510 models?" In my opinion, the most significant difference between the ONE and the GO 910 (or the Garmin nüvi for that matter), is that the ONE doesn't include text-to-speech—a feature that allows the GPS to speak street and highway names and numbers. Beyond that, the GO 510 and 910 both offer wider screens, iPod compatibility, a Bluetooth Handsfree phone interface, and a light sensor. Another important difference between the GO 510 and the ONE is that the 510 has an optional remote control. The GO 910 not only has a remote control but includes all maps of the U.S., Canada, and Europe on a 20GB hard drive that can also store MP3s and pictures, and it has support for audiobooks.

In my road tests, the ONE produced exactly the same routes for our standard test routes as did the GO 910. I prefer the ONE's map display to the Garmin nüvi's. Though the ONE lacks text-to-speech, the voice prompts were clear, easy to understand, and more complete than those of some units I've reviewed.

At $499, the TomTom ONE represents a price breakthrough in the ultraportable GPS product category. Though it lacks some of the bells and whistles of the Garmin nüvi, I think it will find itself attached to a lot of windshields.