I've found that nearly all GPS systems use the same mapping system. I guess the accuracy depends on the quality of the "guts" of the system.
I did see this today:
"If you've picked up a TomTom Global Positioning System device over the past few months, you may have bought more than you bargained for. TomTom International confirmed on Monday that some of its latest Go 910 devices have shipped with a virus preinstalled.
The infected systems were all manufactured during a one-week period around October 2006, TomTom said in a statement posted to its Web site on Monday. The affected systems are running version 6.51 of the TomTom software."