NAVTEQ Maps – It goes without saying that one of the most critical elements of a navigation system is the quality of the maps. The two main suppliers of mapping data are NAVTEQ and Tele Atlas. Both companies have made great strides in this area, but NAVTEQ is still the best choice for North America. As of this writing, NAVTEQ’s maps are more accurate and complete than Tele Atlas.

Number of POI’s – In GPS speak, a "Point of Interest" is a business address saved on the GPS. You can think of the POI database as a pre-loaded yellow pages. Most modern GPS devices come pre-loaded with a Points of Interest database, allowing you to find nearby businesses, such as restaurants, gas stations, hotels, and more. Some less expensive GPS units ship with as few as 750k POI’s, while more expensive units can have up to 8 or even 10 million.

Routing Quality – Even if two GPS units have identical maps installed, they may choose very different routes. That’s because there’s a lot of math and algorithms behind how GPS devices figure out the “best” route to your destination. Is a particular street closed after business hours? Will you arrive only to find your destination is on the left side of a busy street, and you have to make a U-turn or go around the block? Is a given highway known to be congested at this time of day? Is a slightly longer route actually a shorter drive time? These are all factors that GPS manufacturers must tackle, and some do a decidedly better job than others. This is the least well understood aspect of GPS devices, and not something advertised on the box. Be sure to check out our reviews and find out which GPS devices get this right. In general, Garmin does the best job routing, followed by Magellan, and then TomTom.

Ease of Use – How easy is the GPS to use? Could you loan it to a friend and have them use it without any help from you or the instructions? Before buying a GPS, test it out (or read our in-depth reviews on this site) and try to input a destination address, search for a POI (point of interest), and take a look at the map screen. These are the basic functions you’ll want to check out to test ease of use.

Mounting Hardware – It sounds like a minor issue, but all too frequently even the most promising GPS can turn into a lemon when the mounting hardware doesn’t work well. Beenbags, windshield mounts, flex-arms, and adhesive mounting discs are just some of the various methods companies use. Look for a mount that doesn’t vibrate while driving, stays connected to the windshield, and is easy to dock/undock the GPS.

Nice to Have’s – Depending on your budget, features you may want (but aren't critical for basic navigation) include:

• Larger Screen - Many GPS manufacturers now sell 4-inch widescreen GPS units. Though not essential, the wider screen does make things easier to see. Those looking for more portable units that can easily be stowed in a shirt pocket may prefer the more standard 3.5" display size.
• Text-to-Speech - TTS, or Text-to-Speech makes it possible for the GPS to announce actual street names. So instead of hearing "Turn Right Ahead", you'll hear "Turn Right on Main Street".
• Bluetooth - Bluetooth enabled GPS units can be paired to a Bluetooth enabled cell phone for hands-free speakerphone.
• Traffic - Real-time traffic data is available on some GPS units (requires an annual subscription fee of around $65) and allows the GPS to make routing decisions based on current traffic conditions, avoiding accidents and tie-ups. Note that traffic coverage is generally only available in metropolitan areas, so be sure to check the coverage map before subscribing if your GPS supports this feature.
• FM Transmitter - GPS units that have an integrated FM transmitted can route the GPS' audio to your car's stereo speakers via the FM radio (similar to many iPod car adapters that do the same thing). Quality varies, and long trips will almost certainly require you to change channels a few times to find an unused frequency on the FM dial. If your GPS has an audio out jack, you're better off using a cassette adapter or auxiliary audio hookup.

Be sure to check out the GPSmagazine Buyer's Guide for a complete list of all GPS rankings and reviews before buying your next GPS.