And they are as reliable and/or as good as the software you use. there are various GPS add ins for google earth (I've not tried any) or there is software like MS autoroute that will do it too.
I have a Garmin Sat Nav and also have various apps for navigation on my Sony Ericson Smartphone....whilst the apps do work and have many nice features I like the dedicated SE Nuvi much better.....
My honest advice?....buy a new Sat Nav.......the best yiu can afford.