I think a lot also depends on which border guard you get and what their 'feel' for you is. I was in the US a few month ago and got waved through after 30 seconds and a couple of basic questions (might have helped that I was travelling with my wife and daughter). When I was in Canada a few years ago I got called into a separate room and asked a bunch of questions about what I do for a living, what my parents do for a living, how long I was planning to stay, how much money I had with me etc. etc.