I use IAD all the time (typically 15-20 times a year). Most of what people have said is correct:
If arriving on United or Lufthansa and transferring to United, you'll go through immigration and customs at the midfield terminal itself. Follow the purple arrows to 'Transit'.
Everyone else goes to the same arrivals hall in the main terminal. Either the mobile lounges pick you up directly at the aircraft (rare these days, but it does happen) or you get off via a jetway and then follow (sometimes endless) corridors (follow the yellow arrows) to the mobile lounge which will take you to the immigration hall.
I've never been through the transit immigration hall, but the main immigration hall is often hugely backed up because most international flights arrive in the same 2-hour period. You could well be an hour in line, if you're unlucky enough to arrive just after the BA 747 or somebody's full 777. Bring a book. Or you could be really lucky and whiz through in minutes. In the first case, your bags will definitely already be on the carousel by the time you get through, in the second you're likely to have to wait for them.
Either way, you'll HAVE to get your bags off the carousel, take them through customs, and then re-check them. This applies to both the main hall and the transit hall. It's standard policy at ALL US airports.
And then you will have to go through security again to get back airside. Again, standard policy.
I second what others have said about IAD being architecturally beautiful but functionally awful. Recent improvements help, but not enough.