We headed towards the Capitol building around 6pm and the metro was pretty full, but not any fuller than I've seen it at rush hour. We went to a friend's house for a barbecue (yum!) and then walked the mile or so to the Mall around 8:30.
There were a LOT of people there. The lines for the port-a-potties were 20-30 people long. The grass portions of the Mall were completely covered with people sitting and when we arrived the roads cutting across the Mall were just starting to fill up with people standing. We actually had a really good spot on one of the cross streets looking straight at the monument over the people who were sitting!
The fireworks and the canons started at pretty much the same time - and the canons were directly behind us and much louder than the fireworks. We were at the east end of the Mall where there was also a concert and I think the canon blasts were part of ending that? The fireworks were great and people clapped especially for shapes - a couple burst into smiley faces and a few others were stars!
When the fireworks ended, everyone started moving pretty quickly. We were on the edge, so we were able to get off the Mall and start moving north pretty quickly, but we also had to walk west pretty much the whole length of the Mall as we were aiming for the metro red line. As it turned out, we walked more than half way home before we got away from the huge crowds around the metro stations - and by then we figured we might as well finish walking rather than try to stuff ourselves into already-full trains coming from downtown.
Thus we trudged into our apartment just after 11pm, still debating if we would want to do the same thing again next year...