Do you want to use it for navigation or just to fill the space up?
Accuracy is dependant on the receiver, roads and routes on who produces them.
For Europe Garmin is the only game in town.
Most units come with full Europe maps which are accurate and very good.
TomTom sold me a unit that was good for Europe until I updated it and they took off the European maps at the same time, wonderful eh?
For the US anything with the Rockwell chip is a good choice.
If I could have ordered my new Golf without a GPS I would have snapped their hand off, I still might have it out once the warrantee period has elapsed.
Personally a stand alone unit it way better than any permanant installation, regular updates are just a rip off and a way to keep the £, €, or $ rolling in.