I would not call the GPS accurate at all. I was driving in the center of Denver one day and it showed me as driving through lake a good 400 ft off to the left of me. It also frequently shows me as driving through peoples back yards. I think you meant to say it's accurate within 400 feet.
My Garmin GPS is what I call 100% accurate wiithin 5 feet or so,
Then I suggest you get your unit checked, as it seems to be defective.
Since myself and many others are happily using the GPS in the iPhone with high accuracy.
It shows me on google maps with the images enable about 8 feet from where I'm standing.
Last night it showed me on a wooded path I was walking on to hovering between 5-10 feet...
Do you not agree that, that is very accurate?