I have an older GPS NAVI in my car. It cost me $2000 and although for me that is a big wad o cash, I don't regret the investment in the least. I think it is far safer than looking at a Thomas Guide or mapblast directions.
The newer NAVI for my car includes limited voice command input as well. It can even recognize street names although the voice command for it is still clumsy and limited - it does show tremendous promise. Because let's face it - we all input destinations while we drive even though it says not to.
I think improving the voice command and finally accepting BT beamed contacts are required for next upgrade within the next two years. You can transfer contacts between treo's and PPC phones so as long as it can accept one it should the other without issue. This would be wonderful for easily transferring 30 or so contacts I travel to from different directions on a regular basis. The other option would be a usb flash drive with the data exported from outlook.
I paid $2000 5 years ago - today I would expect them to do A LOT more for that same money or cost A LOT less. My $450 PPC phone can respond to over 300 names - untrained by voice command, it runs word, excel, powerpoint, acrobat, it is a phone and for $200 (software, BT GPS UNIT and memory) more works very well as a navigation system.
A NAV system in a new car is about $2000. Personally I don't think they're worth it, especially if you can read a map. To me it's just another distraction like cell phones, putting on make up, shaving, reading a novel, what ever. I"m sure I'm in the minority on this but I enjoy driving and think all this stuff not only detracts from the driving experience but are dangerous. Like most things in life it comes down to whether or not you have the money and if so, is it worth the cost.