Coming in and out of the US through the local major airport -- San Francisco -- my experiences have been mixed coming in. Customs varies between nice and a little bit surly, but never outright so.
However, when I drive or fly to Canada? Canadian customs is amazingly friendly and welcoming -- even when crossing at odd hours like 2am when on a road trip. However, coming back into the states, be it dealing with customs at the border (major and minor crossing) or at the airport, I sometimes ask myself -- "why do I live in this country?"
Customs has a hard job and dealing with a lot of people friendly to rude during the day does take a toll. However, often times, these are the first people that someone coming to visit the US encounters. I would hope for a good first impression, but unfortunately that isn't always the case.
Going to Europe this winter it will be fun to compare and contrast again.
Coming back into Canada, the customs agent was not nearly as friendly. Perhaps we looked a little scruffy after a week of camping, but the questions were more brusque and suspicious.
In Tokyo I was through customs so fast I wasn't sure I was through.
Last time I entered Jamaica I was shaken down. After going back and forth for a few minutes I realized it was a shake down and simply asked him "how much?" $50 later and I was on my way. To make a long story short I had brought a few gifts for friends who were meeting me there and the customs agent claimed I was bringing items into the country to sell. Um yeah, I'm selling 4 towels?
In and out of Costa Rica was interesting. When I went I only took a backpack and surfboards. They didn't say much entering the country and only checked out the surfboard bag. Leaving the country they searched everything multiple times prior to leaving (once at security then again plane side). They were all very polite though.
Admittedly I don't do a lot of international travel but I've never had any problems entering the US have always answered the 1-2 questions with yes/no and "I'm glad to be back."
Given these are some of the first people visitors to this country encounter, I would hope they would be friendly and professional rather than surly and on an power trip.
Every foreign country I have been to (mostly Europe and Canada) has been friendlier and more welcoming than every single instance that I have had in dealing with US customs upon re-entry. And, for the record, I am in the "here's my passport" and answers questions crowd.