My Garmin is, shall we say interesting. It doesnt differentiate between shortest and quickest. Apparently the quickest way to get to Tooting from Leicester is straight through the centre of London.
I find with mine to plan the route first, and have an idea of where you are going, then really use it when you are getting close to the final destination. For that I cant fault it. I travel to lots of town centres around the country (visiting our stores) and its always got me there.
Having said that, I typed in High Street, Solhull on Monday and ended up in some village a few miles away.