I installed one a few months ago and it is OK, but not great. It does a good job of accessing satellites quickly, but there are a lot of significant roads that are not in its database, which is all on CD and must be loaded each time you use the nav function.
It works well showing your present location, as long as you are on a major state route or US highway, and it has a fair amount of local points of interest, like restaurants, etc. It is nice to have the built-in factory look and the screen is bright enough to see easily. The audio sounds good too. All in all it seems to be a quality unit.
To me, its main weakness is in route planning on a trip. It often calculates routes which are obviously not the best way to go, and there is little you can do to modify this without great effort. It will automatically recalculate a new route if you chose to depart from the calculated route. You can set up a trip specifying each individual leg of the route, which is basically chaining several short trips together, but that can be somewhat cumbersome, especially on a long trip. There is no touch screen so the only way to input info is using a small knob to move a cursor around to make various selections.
The CD must be in the drive any time you are using the nav function, so you can not play a music cd at the same time, although you can still use the radio. (This may not be true if you have the remote cd changer installed). Now that you can get one in the $500. range it is worth that.