Deleted my original reply!
I just wanted to post a reply to this old post because it inspired me to visit this water on Saturday. My girlfriend and I hiked up and down stream after arriving at Hemlock Overlook. I don't have waders and the water was still a bit cold for entering, so I fished from shore (a bit tricky). I managed to get two decent-sized bluegill. More importantly, I got a lot of practice casting and learning. I have to say, this spot was a lot more fun than perching on rocks around Fletchers or Chain Bridge where it seems like most of the time I'm just casting blindly into murky water.
I was surprised how quiet it was on a beautiful spring day. If you're looking for a spot to practice in a real nice setting, I'd definitely recommend Bull Run.