99% of my experiences crossing the border between the US and CA have been pretty pleasant though I have had some stand out bad ones. At YYZ flying to Florida my wife and I were pulled out of line into secondary screening. We go up to the desk and they ask us how we can afford to be in the US for 3 months without working. We tell them we have savings they ask if they can see our bank accounts and we say sure then they say "thats ok go take a seat for a minute". They then held us there for over an hour until exactly when our flight was scheduled to depart then called us up and told us we could go. The whole time we had to sit there and watch them sit behind their desks chatting. It was infuriating and we lost our business class seats because the next flight was nearly full.

Ambassador bridge is constantly a positive experience for us but Rainbow bridge my wife(Australian) always gets grilled rudely.

As a green card holder, living in Seattle I travel a lot to Vancouver, BC (15-20 times a year). I haven't had a single problem with US CBP, but Canadian one gives me a hard time every other visit. Though it's an anecdotal evidence of course.