When do you need the impact driver / what do you use it for? I use the drill for all my screwing etc :-)
You will love the impact. It just provides a lot more umph when driving in a screw. One task I find it specifically handy for is toenailing. You don't have to provide hardly any pressure and the thing will drive a screw completely through the stud if you don't pay attention. I have the same drill as you do, the old style, but got the impact with it at the same time.