This isn't appropriate for one-time day travellers and vacationers. Certainly it is a good program but it is no excuse to be assholes to tourists bringing money into either country for what used to be a good time.
Don't you have to give your biometrics though? The privacy / tracking implications seem a bit disturbing.
It is a background check. USA uses your fingerprints for air travel authentication. Canada uses an iris scan for authentication. The authentications are separate but the travel history is shared for visa tracking purposes. It is much less invasive than being queried and documented on varied subject by every singe border agent. The files on you grow much bigger with the unlimited scope of questioning of a border agent. Of course, they can still stop you and ask you details (and do) but I have found once you are on file they stop digging around into your private life because they know you have a background check on file.