I just bought a new car last week with built-in Navigation -- my first experience with a Nav system.
There is no denying the "cool" factor of having it. My car doesn't have voice controlled climate/radio/nav, but I test drove a car that did, and it was pretty nifty to be able to change the radio by calling out the station, or changing the fan speed, etc. Another factor in favor of getting the system is the residual value of the car is a bit higher with the system, if this is a concern to you.
I like having the system. As j8ear mentioned, its simply a real-time, interactive map system. I suppose if you take the same route to work everyday (hehe), rarely leave the vicinity of your house, aren't a real estate agent, or prefer holding a real map in your hands, maybe it's not for you. If you take the occasional (or more) road trip and need the guidance of a map, along with the ability to map out points of interest (gas, atm, food, etc) I would say it is definitely worth having.
Let's face it: its not an essential driving tool. Whether it is worth spending the extra $2k is in the eye (voice?) of the beholder.
You'll probably get by with or without it.
