we went 3 or 4 years and then decided it's much better to sit in our air conditoned apartment and watch it on tv. in terms of crowds, i've been to
disneyland the day after christmas where it was so packed you couldn't move and those crowds will seem light compared to what you will experience on 7/4 in dc.
if you want to see the fireworks from somewhere on the mall, get there by noon, otherwise, it will be hard to get a place to sit. we used to bring food for lunch AND dinner and camp out. if you want a good seat you will have to get there earlier than noon. make sure you have a blanket or something because the crowds have a way of infringing on space claimed.
in years past they have slightly altered the metro routes on the fourth of july. if they do that there will be new maps hung in the stations, so don't get confused if you have a pocket guide or something. also, i would try to completely avoid taking the metro immediately after the fireworks are over no matter what station you get on at. it's crazy everywhere. i would go catch a movie or get some drinks or something, or be prepared for it to take you awhile to get where you are going.
don't even think about driving.
a couple years ago (2001) we got massively DRENCHED when it started downpouring 1 hour or so before the fireworks. i felt like i was at woodstock or something. we got pretty muddy.
the year before that we had record high heat and temps were in the 100's. even with sunscreen we got burned.
basically, it's nice to do once, but i don't think we'll be doing it again anytime soon. i don't like crowds that big.