I've had more problems with the customs agents than I have with the forms. Here's one recent chain of exchanges. I get to the head of a line where two agents are standing. One of them opens my passport.

1st agent (scowling at me): Where have you traveled?
Me: The Philippines
1st agent: What were you doing there?
Me: Just a vacation.
2nd agent (says loudly): HE'S A PIMP!!!!
1st agent: You're going over there (he points to the area where some travelers' luggage is thoroughly checked).

I head to the luggage-check area. The official there has a much more polite demeanor. He also asks where I've been and what I was doing there. Then he says:

Agent: Do you know why you had to come here for a luggage check?
Me: No.
Agent: It's because you only declared one item for entry into the country -- just one box of tea bags.
Me: I see. Well, that's all I bought in the Philippines to bring back. I guess next time I'll buy more and declare more.
Agent: (smiles)

This agent gives everything in my suitcase a thorough search and sends me on my way. Now here's the kicker -- that package of tea I had declared wasn't even in my luggage! I had neglected to pack it at my hotel in Manila. And the customs agent never noticed it wasn't there!

I guess I should add that usually I am treated okay by customs when I come back to the USA. But those first two agents were exceptionally discourteous.