I like my cheap Tom Tom One. It does the basics (navigate you to the right place), locks up to the satellites faster then my friend's Garmin and small enough to fit in my pocket so I don't have to worry about people stealing it out of my car. Also, you can download a (user created) Elise or Exige icon for your car.
I really don't need any of the other fancy features that are on the more expensive models.