U.S customs? No not at all. Unless you have certain things to declare, or items above duty free allowance - which you would indicate on the blue card you're provided on the airplane to fill in. Usually it's just a quick glance at your card, maybe a question or two, and you're done.

Here's a helpful Customs webpage that you can read through in advance if you're concerned about specific items you plan to import: http://www.cbp.gov/xp/cgov/travel/vacation/kbyg/ But if you're a typical tourist bringing back souvenirs from a typical vacation, you've nothing to be concerned with.

The only real hassle I've ever had is waiting on luggage so I can clear customs and waiting in the sometimes long lines to clear immigration/customs!