I've never tried it. Here's some conversations about it at Garden Web that you might find helpful and informative. The first one is an ad for the stuff.
You can also visit this site I found from the Texas Extension Service. Scroll down to L5340 and click on 'View PDF' for 'Maintaining St Augustinegrass Lawns'.
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.