I went to the US for many times and got a 10-year visa. I had no problem until once went there by Greyhound. I forgot to bring my student ID, although I had a return ticket, I was still turned down at the boarder and was sent back, at 1:00am in the morning!

The other time I drove to the boarder. The officer thought I was going to work in the US, he checked my car and trunk very carefully and didn't find any evidence that I will stay in the US. I was passed.

Returning from the US is simple. I had the following conversation with the boarder officer ( a pretty woman):
Canadian Officer: Where do you live?
Me: Tornoto, Ontario
C O: How long have you stayed in the US?
Me: 2 weeks
C O: Any alcohol or tobacco?
Me: No
C O: Thank you, you may go now!
Me: .....