I have a garmin streetpilot with maps that are about 9 years out of date, but its still a cool gizmo. Most of the time I use it in bad weather when its either fog or snow obscured, or simply looking for some place thats in city, but I have never been too before. Which in my opinion is a god send, as it stops me from being a tourist in the car, and actually lets me know a head of time when my turn is coming up.
The newer gps units have really cool features like alerting you ahead of time with regards to traffic and the coolest in my opinion is alerting you to either photo radar traffic lights or known radar locations.
Now what I would like , is for the gps to tell me exactly what lane I am in. My garmin just tells me that I am going up a stretch of road, but having lane information would be a godsend in snow covered roads, specially with those really big ditches.
Second is for GPS units to form some sort of adhoc networking , similar to what wireless does now, so that a car ahead of me would broadcast to my gps, presumeably over cell networks.