Here's some criteria to consider before buying

1. The maps (Canada, United states, North America, Europe)
2. The ability to add new maps (one slot for an SD card is highly recommended)
3. The size of the touch screen (3.5, 4.3, 5.2 inches)
4. The possibility to update the maps easily(forget LG and MIO)
5. Ease of use (Garmin, Magellan, TomTom)
6. Text-To-Speech (TTS), (Tomtom 130s, Tomtom XL 300s,  Tomtom XL 330s,  Tomtom GO 630, 730, 930, Garmin nuvi 255,260,265,275, nuvi série 300, 600,700,800,5000, Magellan Maestro 1212, 1412, 4350, 4370, 5310, Sony, Navigon)
7. Fm transmitter ( Nuvi 700 and 800 seires, nuvi 5000 , tomtom go 730 et 930, maestro 4350 et 4370, nokia 500)




8. The MP3 player 
9. The Optional FM traffic receiver, Already included FM traffic receiver
10. The weather and various MSN services
11. The possibility to use topo maps on your road gps (like nuvi series)
12. The Handsfree calling option with your bluetooth cellphone
13. The speech recognition (série nuvi 800, Tomtom GO 630, 730, 930, Maestro 4350, Nokia 500)
14. The number of Points of interet
15. The Picture viewer with a SD card
16. The possibility to be warned when you're near of custom points of interest (photo radar, other)