On this day, I learned multiple lessons about using a GPS. The first few are obvious – or at least they are now:
Be specific about the address! Winthrop Street is by Logan Airport, and is nowhere near Winthrop Square in Downtown Crossing. There are at least three different Summer Streets in Boston, in three different places.
Check out the GPS map view or the whole sequence of directions before you start out. Then you can pick the one to which you would actually care to go!
And this is the big one of the day: Even though it sounds like a good solution for navigating the one-ways, forget using a GPS in Very Old Boston! The Fanueil Hall area, complete with 200+ year old paved cow paths, twists and turns and small angles, seems to have named (but not labeled) roads that not distinct enough for my Garmin GPS. It freaked out! It went into an incoherent rant that went something like this as I crawled through traffic: “Turn right on Milk Stree.. recalculating turn left on [something] str… recalculating.. recalculating… turn left on .. recalculating…” Well you get the idea.
Jane's Lessons Learned 
That’s when you take out your Blackberry, Bing the address you want, and look at the directions from your current location.