The primary problem isn't that it can't find correct info. It's that it no longer fills in forms automatically. You used to be able to type, for example, the first letter of your name in the Name field of a form; it would complete the field for you. Then you could hit Tab and it would fill in all of the other fields on the form (or at least most of them), using either info from your Address Book entry or from the Other Forms data. I could fill a form out in less than 2 seconds.
Now it requires you to click on a pop-up box or perform some other procedure and it won't fill in forms correctly unless they conform to your Address Book entry (which is not always what you want).
This feature worked perfectly in every Safari version through 5.0.5; they broke it in 5.1 and it's still broken.